BERKELEY 
LIBRARY 

UNIVERSITY  OF 
CALIFORNIA 


SCIENCES 
IJBRARY 


MATTHEW  LIBRARY 


THE  N.  W.  HARRIS  LECTURES  FOR  1914 


N.  W.  partis 

were  founded  in  1906  through  the  generosity  of  Mr.  Norman  Wait  Harris 
of  Chicago,  and  are  to  be  given  annually.  The  purpose  of  the  lecture 
foundation  is,  as  expressed  by  the  donor,  "to  stimulate  scientific  research 
before  the  students  and  friends  of  Northwestern  University,  and  through 
them  to  the  world.  By  the  term  'scientific  research'  is  meant  scholarly  in- 
vestigation into  any  department  of  human  thought  or  effort  without  limi- 
tation to  research  in  the  so-called  natural  sciences,  but  with  a  desire  that 
such  investigation  should  be  extended  to  cover  the  whole  field  of  human 
knowledge." 


1907  Personalism.     Bordon  P.  Browne 

1908  University  Administration.     Charles  W.  Eliot 

1910  The  Age  of  Mammals.     Henry  F.  Osborn 

1911  Democracy  and  Poetry.     Francis  B.  Gummere 

1912  The  Milk  Question.     Milton  J.  Rosenau 

1913  The  Constitution  of  Matter.     Joseph  S.  Ames 


HEREDITY  AND 
ENVIRONMENT 

IN  THE  DEVELOPMENT 

OF  MEN 

BV 

EDWIN  GRANT  CONKLIN 

PROFESSOR  OF  BIOLOGY  :  PRINCETON  UNIVERSITY 


FIFTH   EDITION   REVISED 


PRINCETON 

PRINCETON  UNIVERSITY  PRESS 

LONDON:  HUMPHREY  MILFORD 

OXFORD  UNIVERSITY  PRESS 

1922 


FIRST    EDITION 

Copyright,  1915,  by 
PRINCETON  UNIVERSITY  PRESS 

Published  February,  1915 
Second  Printing,  June,  1915 

REVISED   SECOND   EDITJON 

Copyright,  1916 

Published  May,  1916 

Second  Printing,  August,  1917 

Third  Printing,  March,  1918 

REVISED  THIRD   EDITION 

Copyright,  1919 
..  Published  September,  ,1919 
-Second  Prin'tiKg,  M9'ij:H^  1020 

'    »EVI,SKD   F.OURXH    EDITION' 

' '  '. '  t  ip  Jfriglii:  1922  «V        / 
Published  January',  l9'22 

REVISED    FIFTH    EDITION 

Copyright,  1922 
Published  September,  1922 

TRANSLATIONS 

Japanese,  from  2d  English  Ed. 
Published   September    15,   1916,   Tokyo. 

French,  from   3d  English   Ed. 
Published,   December    1920,   Paris. 


MATTHEW  LIBRARY 


Printed  in  the  United  States 
of  America 


PREFACE  TO  FIRST  EDITION 

The  origin  of  species  was  probably  the  greatest  biological 
problem  of  the  past  century ;  the  origin  of  individuals  is  the  great- 
est biological  subject  of  the  present  one.  The  many  inconclusive 
attempts  to  determine  just  how  species  arose  led  naturally  to  a 
renewed  study  of  the  processes  by  which  individuals  came  into 
existence,  for  it  seems  probable  that  the  principles  and  causes  of 
the  development  of  individuals  will  be  found  to  apply  also  to  the 
evolution  of  races.  As  the  doctrine  of  evolution  wrought  great 
change  in  prevalent  beliefs  regarding  the  origin  and  past  history 
of  man,  so  present  studies  of  development  are  changing  opinions 
as  to  the  personality  of  man  and  the  possibilities  of  improving  the 
race.  The  doctrine  of  evolution  was  largely  of  theoretical  signifi- 
cance, the  phenomena  of  development  are  of  the  greatest  practical 
importance ;  indeed  there  is  probably  no  other  subject  of  such  vast 
importance  to  mankind  as  the  knowledge  of  and  the  control  over 
heredity  and  development.  Within  recent  years  the  experimental 
study  of  heredity  and  development  has  led  to  a  new  epoch  in  our 
knowledge  of  these  subjects,  and  it  does  not  seem  unreasonable  to 
suppose  that  in  time  it  will  produce  a  better  breed  of  men. 

The  lectures  which  comprise  this  volume  were  given  at  North- 
western University  in  February,  1914,  on  the  Norman  W.  Harris 
Foundation  and  were  afterward  repeated  at  Princeton  University, 
f  gladly  take  this  opportunity  of  expressing  to  the  faculties,  stu- 
dents and  friends  of  both  institutions  my  deep  appreciation  of 
their  interest  and  courtesy.  In  attempting  to  present  to  a  general 
audience  the  results  of  recent  studies  on  heredity  and  develop- 
ment, with  special  reference  to  their  application  to  man,  the  au- 
thor has  had  to  choose  between  simplicity  and  sufficiency  of  state- 
ment, between  apparent  dogmatism  and  scientific  caution,  between 
a  popular  and  a  scientific  presentation.  These  are  hard  alterna- 


M  2714 


vl  Preface 

tives,  but  the  first  duty  of  a  lecturer  is  to  address  his  audience  and 
to  make  his  subject  plain  and  interesting,  if  he  can,  rather  than 
to  talk  to  the  scientific  gallery  over  the  heads  of  the  audience. 
In  preparing  the  lectures  for  publication  it  has  not  been  possible 
to  avoid  the  technical  treatment  of  certain  objects,  but  in  the  main 
the  lectures  are  still  addressed  to  the  audience  rather  than  to  the 
scientific  gallery.  Unfortunately  biology  is  still  a  strange  subject 
to  many  intelligent  people  and  its  terminology  is  rather  terrifying 
to  the  uninitiated;  but  it  is  hoped  that  the  glossary  at  the  end  of 
the  volume  may  rob  these  unfamiliar  terms  bf  many  of  their 
terrors. 

The  first  three  chapters  of  this  book  appeared  in  Popular  Sci- 
ence Monthly,  June  to  November  1914,  by  previous  arrangement 
with  the  Princeton  University  Press,  and  a  portion  of  the  last 
chapter  was  first  published  in  Science  in  January  1913.  The  illus- 
trations are  from  many  sources:  *Figures  9,  10,  19,  20,  22,  23, 
26-29,  35,  40-43,  51-54,  58,  72-75,  77,  83  are  original;  Figures 
1-8,  11-18,  21,  24,  25,  30-34,  36-39,  44-47,  49,  55-57,  59-62,  70 
were  redrawn  with  more  or  less  modification  from  original 
sources  which  are  indicated  in  every  case;  the  remaining  figures, 
namely  Figures  48,  50,  63-69,  71,  76,  78-80,  84-96  were  copied 
with  little  or  no  modification  from  various  papers,  photographs 
and  books,  the  original  source  being  given  in  each  case.  The 
writer  wishes  to  express  his  obligation  to  all  authors  and  publish- 
ers upon  whose  works  he  has  drawn,  and  he  thanks  especially  the 
Columbia  University  Press  for  permission  to  use  Figures  48  and 
50,  and  the  Open  Court  Publishing  Company  for  Figures  84-87. 

I  take  this  opportunity  of  thanking  Dr.  W.  E.  Castle  ond  Dr. 
J.  H.  McGregor  for  the  use  of  photographs  which  are  reproduced 
in  Figures  Si,  82  and  96;  and  I  wish  especially  to  thank  my  as- 
sistant, Marguerite  Ruddiman,  for  her  aid  in  preparing  figures 
and  manuscript  for  publication. 
Princeton,  December,  1914 

*  These  figure  numbers  apply  to  the  first  and  second  editions  but  not  to 
later  ones. 


PREFACE  TO  SECOND  EDITION 

The  interest  of  the  public  and  the  kindness  of  the  publishers 
have  made  possible  a  revised  edition  of  this  book  within  a  year 
after  its  first  publication.  This  has  permitted  the  rewriting  of 
certain  passages  which  were  not  well  expressed  and  the  introduc- 
tion of  considerable  new  material  which  will  serve,  it  is  hoped,  to 
further  elucidate  some  of  the  questions  discussed,  as  well  as  to 
give  results  of  more  recent  work.  In  particular  Figs.  4,  5,  9,  10, 
1 8,  19  have  been  added  and  besides  minor  corrections  and  addi- 
tions to  all  the  chapters  considerable  changes  have  been  made  in 
Chapter  III,  section  on  Maternal  Inheritance,  Chapter  IV,  sec- 
tion on  Inheritance  or  Non-Inheritance  of  Acquired  Characters, 
and  Chapter  V,  sections  on  Artificial  Selection,  Origin  of  Muta- 
tions, Past  Evolution  of  Man,  and  Eugenics. 

January,  1916. 

PUBLISHER'S  NOTE. — In  the  second  edition,  second  printing,  August,  1917, 
the  text  was  corrected  by  the  author  so  as  to  embody  some  of  the  results 
of  the  latest  investigations  in  this  subject. 


PREFACE  TO  THIRD  EDITION 

The  exhaustion  of  previous  editions  of  this  book  and  the 
necessity  of  re-setting  the  whole  of  it  presents  the  opportunity 
for  a  rather  thorough  revision  of  the  entire  work.  In  addition 
to  minor  changes  which  have  been  made  in  all  the  chapters  there 
has  been  a  rearrangement  and  enlargement  of  the  material  of  the 
chapter  on  the  "Cellular  Basis  of  Heredity  and  Development" 
and  since  this  is  the  most  technical  chapter  in  the  book  it  is  placed 
after  the  chapter  on  "Phenomena  of  Inheritance"  which  deals  with 
subjects  which  are  more  familiar  to  the  average  reader.  Although 
the  cellular  phenomena  of  heredity  are  less  familiar  and  more 
technical  than  other  aspects  of  this  subject  they  cannot  be  omitted 
or  slighted  if  one  wishes  to  understand  the  most  recent  and  sig- 
nificant discoveries  in  this  field.  Figures  and  descriptions  have 
been  added  of  typical  cells,  of  cell  division,  of  the  origin  and  ma- 
turation of  the  germ  cells,  of  sex  determination  and  especially  of 
the  mechanism  of  heredity  in  that  most  famous  of  all  objects  for 
the  study  of  inheritance,  the  fruit  fly  Drosophila  melanog  aster. 
The  author  is  indebted  to  Professor  Morgan  and  his  associates 
for  permission  to  use  certain  figures  from  their  books  and 
papers  on  this  subject. 

In  this  edition  a  much  stronger  position  has  been  taken  for 
the  "chromosomal  theory"  of  heredity  than  in  former  editions, 
for  this  theory  is  now  so  well  established  that  it  deserves  a  promi- 
nent place  in  even  an  elementary  book. 

In  spite  of  these  additions  the  object  of  keeping  the  presenta- 
tion as  simple  as  possible  has  been  adhered  to  and  those  who  de- 
sire a  more  complete  account  should  consult  the  "Mechanism  of 
Mendelian  Inheritance"  by  Morgan  and  his  associates,  or  "Ge- 
netics in  Relation  to  Agriculture"  by  Babcock  and  Clausen. 

September,  /p/p. 

PUBLISHER'S  NOTE. — The  second  printing  of  this  edition  (March  1920) 
has  been  corrected  and  revised  in  a  few  places. 

viii 


PREFACE  TO  FOURTH  EDITION 

I  had  intended  that  the  previous  revision  of  this  book  should 
have  been  the  last,  but  I  cannot  resist  the  opportunity,  which  is 
again  offered  me  by  the  necessity  of  reprinting  it,  to  make  cer- 
tain corrections  and  alterations,  and  in  particular  to  refer  to  some 
recent  discoveries  of  great  importance.  Knowledge  is  advancing 
so  rapidly  in  the  field  of  Genetics  that  it  is  not  possible  to  keep 
such  a  book  as  this  strictly  up  to  date,  but  at  least  it  should  record 
the  golden  milestones  that  are  passed.  Most  of  these  additions 
will  be  found  in  the  chapter  on  the  Cellular  Basis  of  Heredity  and 
Development;  others  deal  with  the  Phenomena  of  Heredity,  In- 
ternal Secretions,  the  Inheritance  of  Acquired  Characters,  and 
Mutations ;  while  minor  changes  occur  in  all  the  chapters. 

I  am  especially  indebted  to  Dr.  Theophilus  S.  Painter  for  the 
drawing  of  human  chromosomes  shown  in  Figure  56,  and  to  him 
and  to  Professor  Michael  F.  Guyer  for  information  regarding 
the  number  of  chromosomes  in  man.  To  Dr.  Calvin  B.  Bridges 
and  to  Professor  T.  H.  Morgan  I  am  under  obligations  for  infor- 
mation embodied  in  the  new  map  of  Drosophila  chromosomes 
shown  in  Figure  67,  and  I  am  particularly  indebted  to  Professor 
Morgan  for  his  kindness  in  loaning  me  the  figures  of  Drosophila 
mutants  which  are  reproduced  in  Figures  101-103.  Finally  I  am 
glad  to  thank  publicly  my  colleague  Professor  George  H-  Shull 
for  many  valuable  criticisms  and  suggestions. 

Princeton,  December,  1921. 


PREFACE  TO  FIFTH  EDITION 

The  present  edition  of  this  book  follows  so  soon  after  the 
previous  one  that  few  changes  have  been  made  except  in  the 
figures  illustrating  the  text.  Figures  26,  28,  35,  36,  37,  38,  39, 
40,  4ia  47,  61,  62,  66,  74,  82,  86,  87,  95,  96,  97,  98,  99,  104  of  the 
previous  edition  have  been  remade  of  replaced  by  new  figures. 
Again  I  am  especially  indebted  to  Prof esor  Morgan  for  his  kind- 
ness in  furnishing  figures  61,  62,  and  66,  and  for  many  valuable 
suggestions. 

Woods  Hole,  August,  1922. 


CONTENTS 
CHAPTER   I.     FACTS   AND   FACTORS   OF   DEVELOPMENT 

INTRODUCTION 

A.  PHENOMENA  OF  DEVELOPMENT 

I.  DEVELOPMENT  OF  THE  BODY 

1.  The  Germ  Cells 

2.  Fertilization 

3.  Cleavage 

4.  Embryogeny 

5.  Organogeny 

6.  Oviparity  and  Viviparitjr 

7.  Development  of  Functions 

II.  DEVELOPMENT  OF  THE  MIND 

1.  Sensitivity 

2.  Reflexes,  Tropisms,  Instincts 

3.  Memory 

4.  Intellect,  Reason 

5.  Will 

6.  Consciousness 

7.  Parallel  Development  of  Body  and  Mind 

B.  FACTORS  OF  DEVELOPMENT 

1.  Preformation 

2.  Epigenesis 

3.  Endogenesis  and  Epigenesis 

4.  Heredity  and   Environment 

CHAPTER  II.  PHENOMENA  OF  INHERITANCE 
A.  OBSERVATIONS  ON  INHERITANCE 

Individuals  and  their  Characters 
Hereditary  Resemblances  and  Differences 
I.  HEREDITARY  RESEMBLANCES 
I.  Racial  Characters 


xii  Contents 

2.  Individual  Characters 

a.  Morphological  Features 

b.  Physiological  Peculiarities 

c.  Teratological  and  Pathological  Peculiarities 

d.  Psychological  Characters 

II.  HEREDITARY  DIFFERENCES 

1.  New  Combinations  of  Characters 

2.  New  Characters  or  Mutations 

3.  Mutations  and  Fluctuations 

4.  Every  Individual  Unique 

B.  STATISTICAL  STUDY  OF  INHERITANCE 

1.  Galton's  "Law  of  Ancestral  Inheritance" 

2.  Galton's  "Law  of  Filial  Regression" 

C.  EXPERIMENTAL  STUDY  OF  INHERITANCE 

I.  MENDELISM 

1.  Results  of  Crossing  Individuals  with  one  pair  of  contrast- 

ing characters 
Other  Mendelian  Ratios 

2.  Results  of  Crossing  Individuals  with  more  than  one  pair 

of  contrasting  characters 
Dihybrids  and  Trihybrids 

3.  Inheritance  Formulae 

4.  Presence  and  Absence  Hypothesis 

5.  Summary  of  Mendelian  Principles 

a.  The  Principle  of  Unit  Characters 

b.  The  Principle  of  Dominance 

c.  The  Principle  of  Segregation 

II.  MODIFICATIONS  AND  EXTENSIONS  OF  MENDELIAN  PRINCIPLES 

1.  The  Principle  of  Unit  Characters  and  Inheritance  Factors 

2.  Modifications  of  the  Principle  of  Dominance 

3.  The   Principle   of    Segregation 

"Blending"   Inheritance 
Maternal  Inheritance 

III.  MENDELIAN  INHERITANCE  IN  MAN 


Contents  xiii 

CHAPTER  III.    CELLULAR  BASIS  OF  HEREDITY  AND  DEVEL- 
OPMENT 

A.  INTRODUCTORY 

1.  Definitions 

2.  The  Transmission  Hypothesis 

3.  Germinal  Continuity  and  Somatic  Discontinuity 

4.  The  Units  of  Living  Matter 

5.  Heredity  and  Development 

B.  THE  GERM  CELLS 

1.  Fertilization 

2.  Cleavage  and   Differentiation 

3.  The  Origin  of  the  Sex  Cells 

a.  The  Division  Period 

b.  The  Growth  Period 

c.  The  Maturation  Period 

C.  SEX  DETERMINATION 

1.  Chromosomal  Determination 

2.  Environmental  Influence 

D.  THE  MECHANISM  OF  HEREDITY 

I.  THE  SPECIFICITY  OF  GERM  CELLS 
II.  CORRELATIONS  BETWEEN  GERMINAL  AND  SOMATIC  ORGANIZATION 

1.  Nuclear  Inheritance  Theory 

2.  Linkage  of  Characters  and  Chromosomal  Localization 

a.  Sex  Linked  Inheritance 

b.  Other  Cases  of  Linkage 

3.  Cytoplasmic  Inheritance 

a.  Polarity 

b.  Symmetry 

c.  Inverse  Symmetry 

d.  Localization  Pattern 

E.  THE  MECHANISM  OF  DEVELOPMENT 

1.  The  Formation  of  Different  Substances 

2.  Segregation  and  Isolation  of  Substances  in  Cells 

a.  By  Protoplasmic  Movements 

b.  By  Differential  Cell  Divisions 


xiv  Contents 

CHAPTER  IV.    INFLUENCE  OF  ENVIRONMENT 

A.  RELATIVE  IMPORTANCE  OF  HEREDITY  AND  ENVIRON- 

MENT 

1.  Former  Emphasis  on  Environment 

2.  Present  Emphasis  on  Heredity 

3.  Both  Indispensable  to  Development 

B.  EXPERIMENTAL  MODIFICATIONS  OF  DEVELOPMENT 

I.  DEVELOPMENT  STIMULI 

1.  Physical  Stimuli 

2.  Chemical  Stimuli 

II.  DEVELOPMENTAL  RESPONSES 

Dependent  upon    (a)    Nature  of  Organism,    (b)    Nature   of 
Stimulus,    (c)    Stage  of  Development 

1.  Modifications  of  Germ  Cells  before  Fertilization 

2.  During  Fertilization 

3.  After  Fertilization 

C.  FUNCTIONAL  ACTIVITY  AS  A  FACTOR  OF  DEVELOP- 

MENT 

D.  INHERITANCE  OR   NON-INHERITANCE   OF   ACQUIRED 

CHARACTERS 

E.  APPLICATIONS  TO  HUMAN  DEVELOPMENT:   EUTHEN- 

ICS 

CHAPTER  V.    CONTROL  OF  HEREDITY :    EUGENICS 

A.  DOMESTICATED  ANIMALS  AND  CULTIVATED  PLANTS 

I.  INFLUENCE  OF  ENVIRONMENT  IN  PRODUCING  NEW  RACES 
II.  ARTIFICIAL  SELECTION 

1.  The  Methods  of  Breeders 

2.  How  has  Selection  acted? 
III.  METHODS  OF  MODERN  GENETICS 

1.  Mendelian  Association  and  Dissociation  of  Characters 

2.  Mutations 

3.  Causes  of  Mutation 

B.  CONTROL  OF  HUMAN  HEREDITY 

I.  PAST  EVOLUTION  OF  MAN 
II.  CAN  HUMAN  EVOLUTION  BE  CONTROLLED? 

1.  Selective  Breeding  the  only  Method  of  Improving  the  Race 

2.  No  Improvement  in  Human  Heredity  within  Historic  Times 

3.  Why  the  Race  has  not  Improved 


Contents  xv 

III.  EUGENICS 

1.  Possible  and  Impossible  Ideals 

2.  Negative  Eugenical  Measures 

3.  Positive  Eugenical  Measures 

4.  Contributory  Eugenical  Measures 

5.  The  Declining  Birthrate 

CHAPTER  VI.    GENETICS  AND  ETHICS 

I.  THE  VOLUNTARISTIC  CONCEPTION  OF  NATURE  AND  OF  HUMAN 

RESPONSIBILITY 

II.  THE  MECHANISTIC  CONCEPTION  OF  NATURE  AND  OF  PERSON- 
ALITY 

1.  The  Determinism  of  Heredity 

2.  The  Determinism  of  Environment 

III.  DETERMINISM  AND  RESPONSIBILITY 

1.  Determinism   not   Fatalism 

2.  Control  of  Phenomena  and  of  Self 

3.  Birth  and  Growth  of  Freedom 

4.  Responsibility  and  Will 

5.  Our  Unused  Talents 

6.  Self  Knowledge  and  Self  Control 

IV.  THE  INDIVIDUAL  AND  THE  RACE 

1.  The  Conflict  between  the  Freedom 'of  the  Individual  and 

the  Good  of  Society 

2.  Perpetuation  and  Improvement  of   the   Race  the  Hicrhest 

Ethical  Obligation 

REFERENCES 

GLOSSARY 

INDEX 


CHAPTER  I 
FACTS  AND  FACTORS  OF  DEVELOPMENT 


CHAPTER  I 


FACTS  AND  FACTORS  OF  DEVELOPMENT 

INTRODUCTION 

Man's  Place  in  Nature. — One  of  the  greatest  results  of  the  doc- 
trine of  organic  evolution  has  been  the  determination  of  man's 
place  in  nature.  For  many  centuries  it  has  been  known  that  in 
bodily  structure  man  is  an  animal ;  that  he  is  born,  nourished  and 
developed,  that  he  matures,  reproduces  and  dies  just  as  does  the 
humblest  animal  or  plant.  For  centuries  it  has  been  known  that 
man  belongs  to  that  group  of  animals  which  have  backbones,  the 
vertebrates ;  to  that  class  which  have  hair  and  suckle  their  young, 
the  mammals,  and  to  that  order  which  have  grasping  hands,  flat 
nails,  and  thoracic  mammae,  the  primates,  a  group  which  includes 
also  the  monkeys  and  apes.  But  as  long  as  it  was  supposed  that 
every  species  was  distinct  in  its  origin  from  every  other  one,  and 
that  each  arose  by  a  special  divine  fiat,  it  was  possible  to  main- 
tain that  man  was  absolutely  distinct  from  the  rest  of  the  animal 
world  and  that  he  had  no  kinship  to  the  beasts,  though  undoubt- 
edly he  was  made  in  their  bodily  image.  But  with  the  establish- 
ment of  the  doctrine  of  organic  evolution  this  resemblance  be- 
tween man  and  the  lower  animals  has  come  to  have  a  new  sig- 
nificance. The  almost  universal  acceptance  of  this  doctrine  by 
scientific  men,  the  many  undoubted  resemblances  between  man 
and  the  lower  animals,  and  the  discovery  of  the  remains  of  lower 
types  of  man,  real  "missing  links,"  have  inevitably  led  to  the 
conclusion  that  man  also  is  a  product  of  evolution,  that  he  is  a 
part  of  the  great  world  of  living  things  and  not  a  being  who 
stands  apart  in  solitary  grandeur  in  some  isolated  sphere. 

Oneness  of  All  Life- — But  wholly  aside  from  the  doctrine  of 

3 


4  Heredity  and  Environment 

-  'evolution,'  tk£  fact  that  essential  and  fundamental  resemblances 
exist  among  all  kinds  of  organisms  can  not  fail  to  impress 
thoughtful  men.  Life  processes  are  everywhere  the  same  in  prin- 
ciple, though  varying  greatly  in  detail.  All  the  general  laws  of 
life  which  apply  to  animals  and  plants  apply  also  to  man.  This 
is  no  mere  logical  inference  from  the  doctrine  of  evolution,  but 
a  fact  which  has  been  established  by  countless  observations  and 
experiments.  The  essential  oneness  of  all  life  gives  a  direct  hu- 
man interest  to  all  living  things.  If  "the  proper  study  of  man- 
kind is  man,"  the  basic  study  of  man  is  the  lower  organisms 
in  which  life  processes  are  reduced  to  their  simplest  terms,  and 
where  alone  they  may  be  subjected  to  conditions  of  rigid  experi- 
mentation. Upon  this  fundamental  likeness  between  the  life 
processes  of  man  and  those  of  other  animals  are  based  the  won- 
derful advances  in  experimental  medicine,  which  may  be  counted 
among  the  greatest  of  all  the  achievements  of  science. 

Control  of  Development  and  Evolution. — The  experimental 
study  of  heredity,  development  and  evolution  in  forms  of  life  be- 
low man  must  certainly  increase  our  knowledge  of  and  our  con- 
trol over  these  processes  in  the  human  race.  If  human  heredity, 
development  and  evolution  may  be  controlled  to  even  a  slight 
extent  we  may  expect  that  sooner  or  later  the  human  race  will  be 
changed  for  the  better.  At  least  no  other  scheme  of  social  better- 
ment and  race  improvement  can  compare  for  thoroughness,  per- 
manency of  effect,  and  certainty  of  results,  with  that  which  at- 
tempts to  change  the  natures  of  men  by  establishing  in  the  blood 
the  qualities  which  are  desired.  We  hear  much  nowadays  about 
man's  control  over  nature,  though  in  no  single  instance  has  he 
ever  changed  any  law  or  principle  of  nature.  What  he  can  do  is 
to  put  himself  into  such  relations  to  natural  phenomena  that  he 
may  profit  by  them,  and  all  that  can  be  done  toward  the  improve- 
ment of  the  human  race  is  consciously  and  purposively  to  apply 
to  man  those  great  principles  of  development  and  evolution  which 
have  been  at  work,  unknown  to  man,  through  all  the  ages. 


Facts  and  Factors  of  Development 


A.     PHENOMENA   OF   DEVELOPMENT 

Ontogeny  and  Phytogeny. — One  of  the  greatest  and  most  far- 
reaching  themes  which  has  ever  occupied  the  minds 'of  men  is 
the  problem  of  development.  Whether  it  be  the  development  of 
an  animal  from  an  egg,  of  a  race  or  species  from  a  pre-existing 
one,  or  of  the  body,  mind  and  institutions  of  man,  this  problem 
is  everywhere  much  the  same  in  fundamental  principles,  and 
knowledge  gained  in  one  of  these  fields  must  be  of  value  in  each 
of  the  others.  Ontogeny  and  phylogeny  are  not  wholly  distinct 
phenomena,  but  are  only  two  aspects  of  the  one  general  process 
of  organic  development.  The  evolution  of  races  and  of  species 
is  sufficiently  rare  and  unfamiliar  to  attract  much  attention  and 
serious  thought;  while  the  development  of  an  individual  is  a 
phenomenon  of  such  universal  occurrence  that  it  is  taken  as  a 
matter  of  course  by  most  people,  something  so  evident  that  it 
seems  to  require  no  explanation ;  but  familiarity  with  the  fact  of 
development  does  not  remove  the  mystery  which  lies  back  of  it, 
though  it  may  make  plain  many  of  the  processes  concerned.  The 
development  of  a  human  being,  of  a  personality,  from  a  germ 
cell  is  the  climax  of  all  wonders,  greater  even  than  that  involved 
in  the  evolution  of  a  species  or  in  the  making  of  a  world. 

The  fact  of  development  is  everywhere  apparent;  its  principal 
steps  or  stages  are  known  for  thousands  of  animals  and  plants; 
even  the  precise  manner  of  development  and  its  factors  or  causes 
are  being  successfully  explored.  Let  us  briefly  review  some  of 
the  principal  events  in  the  development  of  animals,  and  particu- 
larly of  man,  and  then  consider  some  of  the  chief  factors  and 
processes  of  development.  Most  of  our  knowledge  in  this  field 
is  based  upon  a  study  of  the  development  of  animals  below  man, 
but  enough  is  now  known  of  human  development  to  show  that  in 
all  essential  respects  it  resembles  that  of  other  animals,  and  that 
the  problems  of  heredity  and  differentiation  are  fundamentally 
the  same  in  man  as  in  other  animals. 


6  Heredity  and  Environment 

I.    DEVELOPMENT  OF  THE  BODY 

The  entire  individual — structure  and  functions,  body  and  mind 
— develops  as  a  single  indivisible  unity,  but  for  the  sake  of  clarity 
it  is  desirable  to  deal  with  one  aspect  of  the  individual  at  a  time. 
For  this  reason  we  shall  consider  first  the  development  of  the 
body,  and  then  the  development  of  the  mind. 

I.  The  Germ  Cells. — In  practically  all  animals  and  plants  in- 
dividual development  begins  with  the  fertilization  of  a  female  sex 
cell,  or  egg,  by  a  male  sex  cell,  or  spermatozoon.  The  epigram 
of  Harvey,  "Omne  v'wum  ex  ovo"  has  found  abundant  confirma- 
tion in  all  later  studies.  Both  egg  and  spermatozoon. are  alive 
and  manifest  all  the  general  properties  of  living  things.  How 
little  this  fact  is  appreciated  by  the  public  is  shown  by  the  repeated 
announcements  of  the  newspapers  that  someone  who  has  made  an 
egg  develop  without  fertilization  "has  created  life."  An  egg  or  a 
spermatozoon  is  as  much  alive  as  is  any  other  cell;  as  character- 
istically alive  as  is  the  adult  animal  into  which  it  develops. 

What  is  Life? — It  is  difficult  to  define  life,  as  it  is  also  to  define 
matter,  energy,  electricity,  or  any  other  fundamental  phenomenon, 
but  it  is  possible  to  describe  in  general  terms  what  living  things 
are  and  what  they  do.  Every  living  thing  whatever,  from  the 
smallest  and  simplest  micro-organism  to  the  largest  and  most 
complex  animal,  from  the  microscopic  egg  or  spermatozoon  to  the 
adult  man,  manifests  the  following  distinctive  properties : 

(a)  Protoplasmic  and  Cellular  Organization. — It  contains  pro- 
toplasm, "the  material  basis  of  life,"  which  is  composed  of  the 
most  complex  substances  known  to  chemistry.  Protoplasm  is  not 
a  homogeneous  substance,  but  it  always  exists  in  the  form  of 
cells,  which  are  minute  masses  of  protoplasm  composed  of  many 
distinct  parts,  the  most  important  of  these  being  the  nucleus  and 
the  cell-body  (Fig.  I.) 

The  nucleus  is  a  central  rounded  body  usually  denser  than  the 
.surrounding  cytoplasm  from  which  it  is  separated  by  a  thin  mem- 
brane. It  contains  granules  or  threads  of  a  substance  which  has 


Facts  and  Factors  of  Development 


B 


FIG.  i.  TYPICAL  TISSUE  CELLS  FROM  DIFFERENT  ORGANS.  A,  Epithelial 
cells  from  intestine  of  duck  embryo,  showing  nuclei  with  dark  chromatic 
masses  and  clear  achromatin  surrounded  by  nuclear  membrane,  centrosomes 
as  two  or  three  dark  granules  at  the  free  border  of  the  cells,  and  cytoplasm 
filling  the  cell  body.  B.  Two  nerve  cells,  a.  of  a  ringed  worm,  b.  of  a  fish, 
showing  cell-body  containing  nucleus  and  nucleolus,  many  neurofibrils  and 
in  a  one  nerve  fiber,  in  b  many  processes,  one  of  which  (-J-)  is  the  nerve 
fiber.  C.  Muscle  cell  from  a  round  worm  showing  nucleus  and  cytoplasm 
in  upper  part  of  cell  and  contractile  fibrils  in  lower  part. 


8  Heredity  and  Environment 

a  strong  chemical  affinity  for  certain  dyes  and  hence  is  called 
chromatin;  it  also  frequently  contains  one  or  more  rounded 
bodies  which  look  like  little  nuclei  and  are  called  nucleoli.  The 
chromatin  and  nucleoli  are  imbedded  in  a  substance  which  does 
not  stain  readily  with  dyes  and  which  is  therefore  called  achro- 
matin.  Surrounding  the  nucleus  is  the  substance  of  the  cell-body 
or  cytoplasm  and  in  this  the  various  products  of  differentiation 
such  as  muscle  or  nerve  fibrils,  secretion  products  and  food  sub- 
stances are  found.  The  cytoplasm  often  contains  also  a  centro- 
some  which  is  a  deeply-staining  granule  surrounded  by  radiating 
lines  and  which  is  an  organ  for  causing  intra-cellular  move- 
ments, especially  in  connection  with  the  division  of  the  nucleus 
and  cell  body.  The  nucleus  and  cytoplasm  also  contain  more  or 
less  water  and  inorganic  salts,  and  all  of  these  things  taken  to- 
gether constitute  what  is  known  as  protoplasm  (Fig.  i). 

Protoplasm  is  therefore  organized,  that  is  composed  of  many 
parts  all  of  which  are  integrated  into  a  single  system,  the  cell. 
Higher  animals  and  plants  are  composed  of  multitudes  of  cells, 
differing  more  or  less  from  one  another,  which  are  bound  together 
and  integrated  into  a  single  organism.  Living  cells  and  organisms 
are  not  static  structures  that  are  fixed  and  stable  in  character, 
but  they  are  systems  that  are  undergoing  continual  change. 
They  are  like  the  river,  or  the  whirlpool,  or  the  flame,  which  are 
never  at  two  consecutive  moments  composed  of  the  same  particles 
but  which  nevertheless  maintain  a  constant  general  appearance; 
in  short  they  are  complex  systems  in  dynamic  equilibrium. 

The  principal  physiological  processes  by  which  all  living  things 
maintain  this  equilibrium  are : 

(b)  Metabolism,  or  the  transformation  of  matter  and  energy 
within  the  living  thing  in  the  course  of  which  some  substances  are 
oxidized  into  waste  products,  with  the  liberation  of  energy,  while 
other  substances  are  built  up  into  protoplasm,  each  part  of  every 
cell  converting  food  substances  into  its  own  particular  substance 
by  the  process  of  assimilation. 


Facts  and  Factors  of  Development  9 

(c)  Reproduction,  or  the  capacity  of  organisms  to  give  rise  to 
new  organisms,  of  cells  to  give  rise  to  other  cells,  and  of  parts  of 
cells  to  give  rise  to  similar  parts  by  the  process  of  division. 

(d)  Irritability,  or  the  capacity  of  receiving  and  responding  to 
impinging  energies,  or  stimuli,  in  a  manner  which  is  usually,  but 
not  invariably,  adaptive  or  useful. 


Memb 


FlG.  2.     A   NEARLY  RlPE  HUMAN   OVUM   IN  THE  LlVING  CONDITION.      The 

ovum  is  surrounded  by  a  series  of  follicle  cells  (FC)  inside  of  which  is  the 
clear  membrane  (Memb.)  and  within  this  is  the  ovum  proper  containing 
yolk  granules  (Y)  and  a  nucleus  (N)  embedded  in  a  clear  mass  of  proto- 
plasm. Magnified  500  diameters  (x  500).  (From  O.  Hertwig.)  B,  two 
human  spermatozoa  drawn  to  about  the  same  scale  of  magnification. 
(After  G.  Retzius.) 


io  Heredity  and  Environment 

Germ  Cells  Alive. — Both  the  egg  and  the  sperm  are  living  cells 
with  typical  cell  structures  and  functions,  but  with  none  of  the 
parts  of  the  mature  organism  into  which  they  later  develop.  But 
although  they  do  not  contain  any  of  the  differentiated  structures 
and  functions  of  the  developed  organism,  they  differ  from  other 
cells  in  that  they  are  capable  under  suitable  conditions  of  pro- 
ducing these  structures  and  functions  by  the  process  of  develop- 
ment or  differentiation,  in  the  course  of  which  the  general  struc- 
tures and  functions  of  the  germ  cells  are  converted  into  the  spe- 
cific structures  and  functions  of  the  mature  animal  or  plant. 

Gametes  and  Zygotes. — In  both  plants  and  animals  the  sex 
cells  are  alike  in  many  respects,  though  they  differ  greatly  in  ap- 
pearances. The  female  sex  cells  of  animals  are  called  ova,  the 
male  cells  spermatozoa.  Corresponding  male -and  female  sex  cells 
are  found  in  plants  also.  Collectively  all  kinds  of  sex  cells  are 
called  gametes,  and  the  individual  formed  by  the  union  of  a  male 
and  a  female  gamete  is  known  as  a  zygote,  while  the  cell  formed 
by  the  union  of  egg  and  sperm  is  frequently  called  the  oosperm. 

The  egg  cell  of  animals  is  usually  spherical  in  shape  and  con- 
tains more  or  less  food  substance  in  the  form  of  yolk;  it  varies 
greatly  in  size,  depending  chiefly  upon  the  quantity  of  yolk,  from 
the  great  egg  of  a  bird,  in  which  the  yolk  or  egg  proper  may  be 
hundreds  of  millimeters  in  diameter,  to  the  microscopic  eggs  of 
oysters,  worms,  etc.,  which  may  be  no  more  than  a  few  thou- 
sandths of  a  millimeter  in  diameter.  The  human  ovum  (Fig.  2) 
is  microscopic  in  size  (about  0.2  mm.  in  diameter)  but  it  is  not 
as  small  as  is  found  in  many  other  animals.  It  has  all  the  char- 
acteristic parts  of  any  egg  cell,  and  can  not  be  distinguished  micro- 
scopically from  the  eggs  of  several  other  mammals,  yet  there  is 
no  doubt  that  the  ova  of  each  species  differ  from  those  of  every 
other  species,  and  later  we  shall  see  reasons  for  concluding  that 
the  ova  produced  by  each  individual  are  different  from  those  pro- 
duced by  any  other  individual. 


Facts  and  Factors  of  Development  1  1 

The  sperm,  or  male  gamete,  is  among  the  smallest  of  all  cells 
and  is  usually  many  thousands  of  times  smaller  than  the  egg.  In 
most  animals,  and  in  all  vertebrates,  it  is  an  elongated,  thread- 
like cell  with  an  enlarged  head  which  contains  the  nucleus,  a 
smaller  middle-piece,  and  a  very  long  and  slender  tail  or  flagellum, 
by  the  lashing  of  which  the  spermatozoon  swims  forwards  in  the 
jerking  fashion  characteristic  of  many  monads  or  flagellated 
protozoa.  In  different  species  of  animals  the  spermatozoa  differ 
more  or  less  in  size  and  appearance,  and  there  is  every  reason  to 
believe  that  the  spermatozoa  of  each  species  are  peculiar  in  cer- 
tain respects  even  though  we  may  not  be  able  to  distinguish  any 
structural  difference  under  the  microscope.  The  human  sperma- 
tozoa (Fig.  3)  closely  resemble  those  of  other  primates  but  are 
still  slightly  different,  and  the  conclusion  is  logically  inevitable,  as 
we  shall  see  later,  that  the  spermatozoa  as  well  as  the  ova  of  each 
individual  differ  slightly  from  those  of  every  other  individual. 

2.  Fertilization.  —  If  a  spermatozoon  in  its  swimming  comes  into 
contact  with  a  ripe  but  unfertilized  egg,  the  head  and  middle-piece 
of  the  sperm  sink  into  the  egg  while  the  tail  is  usually  broken  off 
and  left  outside  (Fig.  4).  About  the  time  of  the  entrance  of  the 
spermatozoon  into  the  egg  the  latter  divides  twice,  giving  off  two 
minute  cells  known  as  polar  bodies  which  lie  at  the  upper  or 
animal  pole  of  the  egg.  The  nucleus  in  the  head  of  the  sperm, 
after  it  has  entered  the  egg,  begins  to  absorb  material  from  the 
egg  and  to  grow  in  size  and  at  the  same  time  a  minute  granule,  the 


H 


FIG.  3.    Two  HUMAN  SPERMATOZOA.    A,  showing  the  side  of  the  flattened 
head;  B,  its  edge;  H,  head;  M,  middle-piece;  T,  tail.    (After  G.  Retzius.) 


12 


Heredity  and  Environment 


FIG.  4.  FERTILIZATION  OF  THE  EGGS  OF  STAR-FISH  AND  SEA-URCHIN. 
A-C,  Successive  stages  in  the  entrance  of  a  spermatozoon  into  the  egg  of 
the  star-fish,  Asterias  glacialis.  Only  one  sperm  has  penetrated  the  jelly 
layer  (//)  which  surrounds  the  egg  and  the  peripheral  protoplasm  (pp)  of 
the  egg  protrudes  as  an  entrance  cone  (ec)  to  meet  it.  (After  Fol.) 
D,  Mature  spermatozoon  of  the  sea-urchin,  Toxofrneustes,  showing  head 
(A)  ;  middle-piece  (m)  ;  and  tail  (f).  E-H,  Successive  stages  in  the  pene- 
tration of  the  sperm  nucleus  (  $  N)  and  centrosome  (#C)  into  the  egg  of 
Toxopneustes.  I-L,  Stages  in  the  approach  of  the  sperm  nucleus  ($N) 
to  the  egg  nucleus  ($AO,  and  in  the  division  of  the  sperm  centrosome 
( $  C)  and  the  formation  of  the  first  cleavage  spindle.  (D-L  after 
Wilson.) 


Facts  and  Factors  of  Development  13 

.  \ 

centrosome,  appears,  either  from  the  middle-piece  or  from  the 
head  of  the  sperm,  and  radiating  lines  run  out  from  the  centro- 
some into  the  substance  of  the  egg.  The  sperm  nucleus  and  cen- 
trosome then  approach  the  egg  nucleus  and  ultimately  the  two 
nuclei  come  to  lie  side  by  side  (Fig.  4).  Usually  when  one 
spermatozoon  has  entered  an  egg  all  others  are  barred  from  en- 
tering, probably  by  some  change  in  the  surface  layer  of  the  egg  or 
in  the  chemical  substances  given  out  by  the  egg. 

Oosperm  or  Zygote  a  Double  Being. — This  union  of  a  single 
spermatozoon  with  an  egg  is  known  as  fertilization.  Whereas 
egg  cells  are  usually,  but  not  invariably,  incapable  of  development 
unless  fertilized,  there  begins,  immediately  after  fertilization,  a 
long  series  of  transformations  and  differentiations  of  the  ferti- 
lized egg  which  leads  to  the  development  of  a  complex  animal,  even 
of  a  person.  In  the  fusion  of  egg  and  sperm  a  new  individual, 
the  oosperm,  comes  into  being.  The  oosperm,  formed  by  the  union 
of  the  two  sex  cells,  is  really  a  double  cell,  since  parts  of  the 
egg  and  sperm  never  lose  their  identity,  and  the  individual  which 
develops  from  this  oosperm  is  a  double  being;  even  in  the  adult 
man  this  double  nature  of  every  cell,  caused  by  the  union  of  egg 
and  sperm,  is  never  lost. 

A  New  and  Distinct  Individual. — In  by  far  the  larger  number 
of  animal  species  the  oosperm,  either  just  before  or  shortly 
after  fertilization,  is  set  free  to  begin  its  own  individual  existence, 
and  in  such  cases  it  is  perfectly  clear  that  the  fertilization  of  the 
egg  marks  the  beginning  of  the  new  individual.  But  in  practi- 
cally every  class  of  animals  there  are  some  species  in  which  the 
fertilized  egg  is  retained  within  the  body  of  the  mother  for  a 
varying  period  during  which  development  is  proceeding.  In 
such  cases  it  is  not  quite  evident  that  the  new  individual  comes 
into  being  with  the  fertilization  of  the  egg;  rather  the  moment  of 
birth,  or  separation  from  the  mother,  is  generally  looked  upon  as 
the  beginning  of  the  individual's  existence.  And  yet  in  all  cases 


Heredity  and  Environment 


FIG.  5.  FIRST  CLEAVAGE  OF  THE  EGG  OF  THE  SEA-URCHIN,  Echinus  micro 
tubcrculatus.  A,  Nuclear  division  figure,  or  mitotic  spindle,  with  a  centro- 
some  at  each  pole  and  with  the  chromosomes  from  the  egg  and  sperm 
nuclei  at  the  equator  of  the  spindle.  B  and  C,  Later  stages  showing  the 
separation  of  the  daughter  chromoses  from  one  another  and  their  move- 
ment toward  the  two  poles  of  the  spindle.  D  and  E,  Still  later  stages 
showing  the  swelling  of  the  chromosomes  and  their  fusion  to  form  nuclear 
vesicles.  F,  Complete  division  of  egg  into  two  cells,  each  containing  one 
daughter  nucleus  and  centrosome.  (After  Boveri.) 


Facts  and  Factors  of  Development  15 

the  new  individual  is  always  distinguishable  from  the  body  of  the 
mother  since  there  is  no  protoplasmic  connection  between  the  two. 
In  mammals  generally,  including  also  the  human  species,  not  a 
strand  of  protoplasm,  not  a  nerve  fiber,  not  a  blood  vessel  passes 
over  from  the  mother  to  the  embryo ;  the  latter  is  from  the  mo- 
ment of  fertilization  onward  a  distinct  individual  with  particu- 
lar individual  characteristics,  and  this  is  just  as  true  of  viviparous 
animals  in  which  the  egg  undergoes  a  part  of  its  development 
within  the  body  of  the  mother  as  it  is  of  oviparous  ones  in  which 
the  eggs  are  laid  before  development  begins. 

•The  fertilized  egg  of  a  star-fish  or  frog  or  man  is  not  a  dif- 
ferent individual  from  the  adult  form  into  which  it  develops, 
rather  it  is  a  star-fish,  a  frog,  or  a  human  being  in  the  one-celled 
stage.  This  fertilized  egg  fuses  with  no  other  cells,  it  takes  into 
itself  no  living  substance,  but  manufactures  its  own  protoplasm 
from  food  substances ;  it  receives  food  and  oxygen  from  without 
and  it  gives  out  carbonic  acid  and  other  waste  products;  it  is 
sensitive  to  certain  alterations  in  the  environment  such  as  ther- 
mal, chemical  and  electrical  changes — it  is,  in  short,  a  distinct 
living  thing,  an  individual  or  person.  Under  proper  environmen- 
tal conditions  this  fertilized  egg  cell  develops,  step  by  step,  with- 
out the  addition  of  anything  from  the  outside  except  food,  water, 
oxygen,  and  such  other  raw  materials  as  are  necessary  to  the  life 
of  any  adult  animal,  into  the  immensely  complex  body  of  a  star- 
fish, a  frog,  or  a  man.  At  the  same  time,  from  the  relatively 
simple  reactions  and  activities  of  the  fertilized  egg  there  develop, 
step  by  step,  without  the  addition  of  anything  from  without  except 
raw  materials  and  environmental  stimuli,  the  multifarious  activi- 
ties, reactions,  instincts,  habits,  and  intelligence  of  the  mature 
animal. 

Is  not  this  miracle  of  development  more  wonderful  than  any 
possible  miracle  of  creation?  And  yet  as  one  watches  this  mar- 
vellous process  by  which  the  fertilized  egg  grows  into  the  embryo, 
and  this  into  the  adult,  each  step  appears  relatively  simple,  each 


16  Heredity  and  Environment 

perceptible  change  is  minute ;  but  the  changes  are  innumerable  and 
unceasing  and  in  the  end  they  accomplish  this  miracle  of  trans- 
forming the  fertilized  egg  cell  into  the  fish  or  frog  or  man — a 
thing  which  would  be  incredible  were  it  not  for  the  fact  that  it 
has  been  seen  by  hundreds  of  observers  and  can  be  verified  at 
any  time  by  those  who  will  take  the  trouble  to  study  the  process 
for  themselves. 

3.  Cell  Division. — After  fertilization  the  first  step  in  devel- 
opment is  the  cleavage  or  division  of  the  egg.  This  is  in  the 
main  like  any  typical  cell  division  and  since  the  details  of  this 
process  are  of  extraordinary  interest  in  the  study  of  the  mechan- 
ism of  heredity  and  development  it  is  desirable  to  give  at  once 
a  rather  detailed  account  of  the  way  in  which  the  nucleus  and 
cell-body  divide. 

a.  Mitosis  or  Indirect  Division  of  the  Nucleus. — It  was  once 
supposed  that  both  the  nucleus  and  the  cell-body  divide  by  a 
simple  process  of  constriction  or  direct  division,  but  it  is  now 
known  that  the  nucleus  rarely  divides  in  this  manner,  and  that 
the  nuclei  of  germ  cells  never  do  so.  On  the  contrary  the  nu- 
cleus almost  always  divides  by  a  complex  process  known  as 
mitosis  or  indirect  division  (Figs.  6  and  7).  During  this  process 
the  chromatin  granules  of  the  "resting"  nucleus  become  arranged 
in  lines  like  beads  on  a  thread  (Fig.  8)  ;  these  threads,  which  are 
called  chromosomes,  are  at  first  long  and  slender  and  much 
coiled,  but  afterwards  they  grow  shorter,  thicker  and  straighter 
and  it  can  then  be  seen  that  in  each  species  of  animal  or  plant 
there  is  a  definite  and  constant  number  of  these  threads  or  chro- 
mosomes (Fig.  6,  A-D)  ;  this  number  varies  from  2  to  200  in  dif- 
ferent species  of  animals,  the  most  usual  number  being  some- 
where between  10  and  30,  but  so  far  as  is  known  each  species  has 
a  constant  number  of  chromosomes  in  every  cell  of  the  body. 

The  nucleolus  and  the  nuclear  membrane  then  disappear,  the 
chromosomes  move  into  the  equator  of  the  cell  forming  the  equa- 
torial plate  (Fig.  6,  F)  and  each  one  splits  lengthwise  into  two 


Facts  and  Factors  of  Development  17 

daughter  chromosomes  which  move  apart  toward  the  two  poles  of 
the  cell  (Fig.  7,  G,  H)  where  all  the  daughter  chromosomes  come 
together  to  form  the  two  daughter  nuclei.  The  cell  body  then 
divides  by  a  process  of  constriction  into  two  daughter  cells  (Fig. 

7,  /,  /)• 

The  formation,  splitting  and  separation  of  the  chromosomes 

is  the  most  constant  and  characteristic  feature  of  indirect  nuclear 
division,  but  there  are  other  important  features  which  must  now 
be  mentioned.  In  all  animals  and  in  many  of  .the  lower  plants 
there  is  present  in  the  cell-body  just  outside  the  nuclear  mem- 
brane a  small  deeply-staining  granule,  the  centrosome,  which  is 
usually  surrounded  by  radiating  lines.  In  the  early  stages  of 
mitosis  this  granule  divides  into  two  which  move  apart  until  they 
come  to  lie  on  opposite  sides  of  the  nucleus  (Fig.  6,  A-C).  When 
the  nuclear  membrane  dissolves  the  radiating  lines  which  sur- 
round these  two  centrosomes  increase  greatly  in  length  forming 
two  asters  and  those  rays  which  run  through  the  nuclear  area 
constitute  a  spindle  with  the  chromosomes  in  its  equator  and  the 
centrosomes  at  its  two  poles  (Fig.  6,  D-F).  Later  the  chromo 
somes  move  along  the  spindle  toward  its  poles  where  the  daugh- 
ter nuclei  are  formed.  The  centrosomes,  asters  and  spindle, 
known  collectively  as  the,  amphiaster,  constitute  an  apparatus  for 
the  accurate  separation  of  the  daughter  chromosomes  and  for  the 
division  of  the  cell-body. 

The  chromosomes  are  most  compact  and  deeply-staining  at 
the  metaphase  or  equatorial  plate  stage  of  division;  after  they 
have  moved  to  the  poles  of  the  spindle  they  begin  to  absorb 
achromatin  from  the  surrounding  plasma  thus  swelling  up  and 
becoming  chromosomal  vesicles  with  clear  contents  and  chromatic 
walls  (Fig.  8,  E,  F).  These  vesicles  then  continue  to  enlarge  and 
their  chromatin  takes  the  form  of  threads  or  granules.  After 
the  formation  of  the  daughter  nuclei  the  different  vesicles  are  so 
closely  pressed  together  that  it  is  usually  impossible  to  see  the 
partition  walls  between  them;  however  in  several  different  ani- 


i8 


Heredity  and  Environment 


mals  and  plants  the  chromosomal  vesicles  are  recognizable  even 
in  the  resting  nucleus  (Fig.  8,  G),  and  in  every  organism  the  same 
number  of  chromosomes,  having  the  same  relative  shapes  and 


FIG.  6. 


FIGS.  6,  7.  DIAGRAMS  OF  SUCCESSIVE  STAGES  IN  THE  DIVISION  OF  A  CELL  BY 
MITOSIS.  A,  Cell  with  "resting"  nucleus  and  centrosome  (c)  ;  B-E,  Early 
stages  of  division  during  which  the  chromatin  takes  the  form  of  short 
thick  threads,  the  chromosomes  (C,  D,  E)  while  the  centrosome  gives 


'Facts  and  Factors  of  Development 


19 


sizes,  come  out  of  a  nucleus  at  the  time  of  division  as  went  into 
it  at  the  preceding  mitosis,  each  new  chromosome  coming  out  of  a 
chromosomal  vesicle  (Fig.  8,  A\  B,  C).  Whenever  one  can  trace 


FIG.  7. 

rise  to  a  spindle  (a)  with  astral  radiations  at  its  poles;  F,  Middle  stage 
of  division  in  which  the  chromosomes  lie  in  the  equator  of  the  spindle 
forming. the  equatorial  plate  (ep)  ;  G,  H,  Stages  showing  the  splitting  of 
each  chromosome  and  the  movement  of  the  halves  toward  the  poles  of 
the  spindle;  ep,  Equatorial  plate;  if,  Interzonal  filaments;  n,  Nucleolus: 
I,  Complete  separation  of  daughter  chromosomes  and  formation  of  daugh- 
ter nuclei;  beginning  of  division  of  cell  body;  /,  Complete  separation  of 
daughter  cells  and  return  of  nucleus  and  centrosome  to  resting  condi- 
tion. (After  Wilson.) 


20 


Heredity  and  Environment 


FIG.  8.  SUCCESSIVE  STAGES  OF  MITOSIS  IN  THE  CLEAVAGE  OF  THE  EGG  OF  A  FISH 
(Fundulus)  showing  that  new  chromosomes  are  formed  inside  of  old  ones  (chromosomal 
vesicles),  that  chromosomes  or  chromosomal  vesicles  persist  from  one  division  to  the 
next  and  that  even  the  "resting"  nucleus  is  composed  of  chromosomal  vesicles.  A,  nu- 
cleus at  beginning  of  mitosis,  having  shrunk  from  dotted  outline  and  showing  chromo- 
somal vesicles  containing  chromatin  granules;  B,  each  chromosomal  vesicle  contains 
granules  or  chromosomeres  which  are  condensing  to  form  a  chromosome;  C,  amphiaster 
showing  faint  outlines  of  the  chromosomal  vesicles  with  their  cntained  chromosomes-; 
D,  amphiaster  showing  each  chromosome  beginning  to  split  and  the  chromosomes  divid- 
ing; E,  late  phase  of  division  showing  daughter  chromosomes  at  the  poles  of  the 
spindle  and  each  chromosome  becoming  vesicular;  F,  still  later  phase,  each  chromosome 
a  vesicle  containing  chromatin  granules;  G,  daughter  nucleus  showing  chromosomal 
vesicles  containing  scattered  chromatin  granules.  (After  Richards.) 


Facts  and  Factors  of  Development  21 

individual  chromosomes  or  chromosomal  vesicles  through  the 
resting  stage  it  is  certain  that  every  chromosome  preserves  its 
individuality  or  identity,  and  even  where  this  cannot  be  done  the 
fact  that  the  same  number  of  chromosomes,  having  the  same 
peculiarities  of  shape  and  size  come  out  of  a  nucleus  as  went  into 
it,  is  evidence  that  here  also  each  chromosome  has  preserved  its 
identity. 

b.  Cleavage  of  the  Egg. — After  the, entrance  of  the  spermato- 
zoon into  the  egg  the  sperm  nucleus  moves  toward  the  egg  nu- 
cleus until  the  two  meet  when  they  divide  by  mitosis  (Figs.  4 
I-L,  5  A-F).  The  centrosome,  which  usually  accompanies  the 
sperm  nucleus  in  its  passage  through  the  egg,  divides  and  forms 
a  spindle-shaped  figure  with  astral  radiations  at  its  two  poles 
(Fig.  4).  The  chromatin,  or  stainable  substance  of  the  egg 
and  sperm  nuclei,  takes  the  form  of  threads  or  chromosomes 
(Fig.  5).  Each  chromosome  then  splits  lengthwise,  its  two  halves 
moving  to  opposite  ends  of  the  spindle,  where  the  daughter  chro- 
mosomes fuse  together  to  form  the  daughter  nuclei.  In  this  way 
the  chromatin  of  the  egg  and  sperm  nuclei  is  exactly  halved. 

After  the  germ  nuclei  have  divided  in  this  manner  the  entire 
egg  divides  by  a  process  of  constriction  into  two  cells  (Fig. 
5  F).  This  is  the  beginning  of  a  long  series  of  cell  divisions, 
each  of  them  essentially  like  the  first,  by  which  the  egg  is  sub- 
divided successively  into  a  constantly  increasing  number  of  cells. 
During  the  earlier  divisions  there  is  little  or  no  increase  in  the 
volume  of  the  egg,  consequently  successive  generations  of  cells 
continually  grow  smaller  (Figs.  9-11).  This  process  is  known 
as  the  cleavage  of  the  egg,  and  by  it  the  egg  is  not  only  split  up 
into  a  considerable  number  of  small  cells,  but  a  much  more  im- 
portant result  is  that  the  different  kinds  of  protoplasm  in  the  egg 
become  isolated  in  different  cleavage  cells,  so  that  these  sub- 
stances can  no  longer  freely  commingle.  The  cleavage  cells,  in 
short,  come  to  contain  different  kinds  of  substance,  and  thus  to 
differ  from  one  another.  The  differentiations  of  the  cleavage 


22 


Heredity  and  Environment 
A  B 


FIG.  9.  SUCCESSIVE  STAGES  IN  THE  CLEAVAGE  AND  GASTRULATION  OF  Am- 
phioxus.  A,  one  cell;;  B,  two  cells;  C  and  D,  four  cells;  E,  eight  cells; 
F,  sixteen  cells;  G,  blastula  stage  of  about  ninety-six  cells;  H,  section 
through  the  same  showing  the  cleavage  cavity;  /,  blastula  seen  from  the 
left  side  showing  three  zones  of  cells,  viz.,  an  upper  clear  zone  of  ectoderm, 
a  middle-  (faintly  shaded)  zone  of  mesoderm  and  a  lower  (deeply  shaded) 
zone  of  endoderm  cells;  /,  section  through  the  same  showing  these  three 
types  of  cells;  K  and  L,  successive  stages  in  the  infolding  of  the  endo- 
derm ;  cells  indicated  as  in  the  preceding  figure.  The  polar  body  is  shown 
at  the  upper  pole,  a,  anterior ;  p,  posterior ;  v,  ventral ;  d,  dorsal ;  be,  blas- 
toccel;  gc,  gastroccel. 


Facts  and  Factors  of  Development  23 

cells  appear  much  earlier  in  some  forms  than  in  others,  but  in  all 
cases  such  differentiations  appear  during  early  or  late  cleavage 
(Figs.  9-1 1 ). 

4.  Embryogeny. — From  this  stage  onward  the  course  of  de- 
velopment differs  in  different  classes  of  animals  to  such  an  extent 
that  it  is  difficult  to  formulate  any  general  description  which  will 
apply  to  all  of  them.     Usually  the  many  cleavage  cells  form  a 
hollow  sphere,  the  blastula  (Figs.  9,  u,  //),  and  this  in  turn  be- 
comes a  gastrula   (Figs.  9,  n,  K),  in  which  at  first  two,  and 
later  three,  groups  or  layers  of  cells  may  be  recognized ;  the  outer 
layer,  which  is  formed  from  cells  nearest  the  upper  pole  of  the 
egg,  is  the  ectoderm ;  the  inner  layer,  or  endoderm,  is  formed  from 
cells  nearest  the  lower  pole ;  a  middle  layer,  or  group  of  cells,  the 
mesoderm,  is  formed  from  cleavage  cells  which  in  vertebrates  lie 
between  the  upper  and  lower  poles  (Fig.  u,  m). 

5.  Organogeny. — 'By    further    differentiation   of   the    cells   of 
these  layers  and  by  dissimilar  growth  and  folding  of  the  layers 
themselves  the  various  organs  of  the  embryo  begin  to  appear. 
From  the  ectoderm  are  formed  the  outer  layer  of  the  skin  and  the 
whole  nervous  system ;  from  the  endoderm  arise  the  lining  of  the 
alimentary  canal  and  its  outgrowths ;  from  the  mesoderm  come,  in 
whole  or  in  part,  the  skeletal,  muscular,  vascular,  excretory,  and 
reproductive  systems.    In  vertebrates  the  nervous  system  appears 
as  a  plate  of  rather  large  ectoderm  cells  (Fig.  n,  n)  ;  this  plate 
rolls  up  at  its  sides  to  form  a  groove  and  then  a  tube ;  and  by  en- 
largement of  certain  portions  of  this  tube  and  by  foldings  and 
thickenings  of  its  walls  the  brain  and  spinal  cord  are  formed 
(Fig.  n,  K,  L;  13,  C,  D).    The  retina  or  sensory  portion  of  the 
eye  is  formed  as  an  outgrowth  from  the  fore  part  of  the  brain 
(Fig.  13,  D)  ;  the  sensory  portion  of  the  ear  comes  from  a  cup- 
shaped  depression  of  the  superficial  ectoderm  which  covers  the 
hinder  portion  of  the  head  (Fig.  13,  E  and  F).    The  back-bone 
begins  to  appear  as  a  delicate  cellular  rod  (Fig.  n,  c),  which  then 
in   higher   vertebrates    becomes    surrounded    successively   by    a 


Heredity  and  Environment 
.V 


FIGS.  10,  ii.  DIAGRAMS  OF  FROG'S  EGGS  SHOWING  THE  RELATIONS  OF  THE 
AXES  AND  SUBSTANCES  OF  THE  EGG  TO  THE  AXES  AND  PRINCIPAL  ORGANS  OP 
THE  EMBRYO.  All  eggs  viewed  from  right  side,  polar  bodies  above;  A, 
anterior;  P,  posterior;  D,  dorsal;  V,  ventral;  s,  spermatozoon;  $N,  sperm 
nucleus,  $N,  egg  nucleus;  m,  mesodermal  crescent  where  mesoderm  will 
form;  c  and  n,  gray  crescent,  where  chorda  (c)  and  nervous  system  (n) 
will  form;  en,  area  of  endoderm;  area  around  polar  bodies  will  form  ecto- 
derm of  skin ;  sc,  segmentation  cavity ;  bp,  blastopore ;  E,  enteron ;  M, 
region  of  mouth. 


Facts  and  Factors  of  Development 


FIG.  10.  SURFACE  VIEWS  OF  ENTIRE  EGGS.  A,  before  entrance  of  sper- 
matozoon; B,  just  after  entrance  of  sperm;  C,  union  of  egg  and  sperm 
nuclei ;  D,  2-cell  stage ;  £,  4-cell  stage ;  F,  8-cell  stage. 

FIG.  ii.  SECTIONS  IN  MEDIAN  PLANE  OF  EMBRYOS.  G,  i6-32-cell  stage; 
H,  blastula;  /,  early  gastrula;  /,  late  ga&trula;  K,  early  embryo,  L,  late 
embryo. 


26  Heredity  and  Environment 

fibrous,  a  cartilaginous,  and  a  bony  sheath.  And  so  one  might 
go  on  with  a  description  of  all  the  organs  of  the  body,  each  of 
which  begins  as  a  relatively  simple  group  or  layer  of  cells,  which 
gradually  become  more  complicated  by  a  process  of  growth  and 
differentiation,  until  these  embryonic  organs  assume  more  and 
more  the  mature  form. 

6.  Oviparity  and  Viviparity. — This  very  brief  and  general 
statement  of  the  manner  of  embryonic  development  applies  to  all 
vertebrates,  man  included.  There  are  many  special  features  of 
human  development  which  are  treated  at  length  in  works  on  em- 
bryology, but  which  need  not  detain  us  here  since  they  do  not 
affect  the  general  principles  of  development  already  outlined. 
In  one  regard  the  development  of  the  human  being  or  of  almost 
any  mammal  is  apparently  very  different  from  that  of  a  bird  or 
frog  or  fish,  viz.,  in  the  fact  that  in  the  former  the  embryonic 
development  takes  place  within  the  body  of  the  mother  whereas 
in  the  latter  the  eggs  are  laid  before  or  soon  after  fertilization. 
In  man,  after  the  cleavage  of  the  egg,  a  hollow  vesicle  is  formed, 
which  becomes  attached  to  the  uterine  walls  by  means  of  processes 
or  villi  which  grow  out  from  it  (Fig.  12,  D,  E,  F)  while  only  a 
small  portion  of  the  vesicle  becomes  transformed  into  the  embryo. 
There  is  thus  established  a  connection  between  the  embryo  and  the 
uterine  walls  through  which  nutriment  is  absorbed  by  the  embryo. 
And  yet  this  difference  is  not  a  fundamental  one  for  in  different 
animals  there  are  all  stages  of  transition  between  these  two  modes 
of  development.  While  in  most  fishes,  amphibians  and  reptiles  the 
eggs  are  laid  at  the  beginning  of  development  and  are  free  and 
independent  during  the  whole  course  of  ontogeny,  there  are  certain 
species  in  each  of  these  classes  in  which  the  development  takes 
place  within  the  body  of  the  mother.  Even  in  birds  a  portion  of 
the  development  takes  place  within  the  body  of  the  female  before 
the  eggs  are  laid,  and  there  are  mammals  (monotremes)  which 
lay  eggs,  while  in  others  (marsupials)  the  young  are  born  in  a  very 
imperfect  condition. 

Mother  always  Distinct  from  Child. — These  facts  indicate  that 


Facts  and  Factors  of  Development 


27 


there  is  no  fundamental  difference  between  oviparity  and  vivi- 
parity.  In  the  latter  the  union  between  the  embryo  and  the 
mother  is  a  nutritive  but  not  a  protoplasmic  one.  Blood  plasma 
passes  from  one  to  the  other  by  a  process  of  soakage,  and  the  only 
maternal  influences  which  can  affect  the  developing  embryo  are 
such  as  may  be  conveyed  through  the  blood  plasma  and  are  chiefly 
nutritive  in  character/  Careful  studies  have  shown  that  sup- 


FIG.  12.    DIAGRAMS  SHOWING  THE  EARLY  DEVELOPMENT  OF  THE  HUMAN 
OOSPERM.    A,  cleavage  stage  which  has  just  come  into  the  uterus;  B  and 

C,  blastodermic  vesicles  embedded  in  the  mucous  membrane  of  the  uterus ; 

D,  E  and  F,  longitudinal  sections  of  later  stages,  the  anterior  and  poster- 
ior poles  being  marked  by  the  axis  a  p.  In  C  cavities  have  appeared 
in  the  ectoderm,   endoderm   and  mesoderm.     D,  villi   forming  from  the 
trophoblast  (nutritive  layer,  tr}  ;  black  indicates  ectoderm  (ect)  ;  oblique 
lines,  endoderm;  few  stipples,  mesoderm;  V,  villi;  am,  amnion;  ys,  yolk 
sac;  n,  neurenteric  canal;  x  25.     (After  Keibel.) 


28 


Heredity  and  Environment 


FIG.  13.  A-H,  successive  stages  in  the  early  development  of  the  human  embryo;  A, 
blastodermic  vesicle  showing  primitive  axis  in  embryonic  area,  age  unknown;  By  blasto- 
dermic  vesicle  attached  to  uterine  wall  at  the  posterior  pole,  showing  neural  groove,  age 
unknown;  C,  later  stage  in  which  the  neural  folds  are  closing  and  five  pairs  of  somites 
have  appeared,  age  ten  to  fourteen  days;  D,  stage  of  fourteen  somites  showing  enlarge- 
ments of  the  neural  folds  at  the  anterior  end  which  will  form  the  brain,  age  fourteen 
to  sixteen  days;  E  and  F,  later  stages,  the  latter  with  twenty-three  somites  and  three 
visceral  clefts;  the  ear  shows  as  a  depression  at  the  dorsal  angle  of  the  second  cleft; 
G,  embryo,  of  thirty-five  somites,  showing  eye,  branchial  arches  and  limb  buds;  H,  em- 
bryo >of  thirty-six  somites  showing  nasal  pit,  eye,  branchial  arches  and  clefts,  limb  buds 
and  heart.  (After  Keibel.) 


Facts  and  Factors  of  Development 
A  B 


29 


FIG.  14.  A,  human  embryo  of  forty-two  somites,  age  twenty-one  days; 
B,  embryo  of  about  four  weeks;  C,  still  older  embryo  showing  the  begin- 
nings of  the  formation  of  digits;  D,  embryo  of  about  two  months;  C  and 
D  are  drawn  on  a  smaller  scale  than  A  and  B.  (After.Keibel.) 


30  Heredity  and  Environment 

posed  "maternal  impressions"  of  the  physical,  mental,  or  emo- 
tional conditions  of  the  mother  upon  the  unborn  child  have  no 
existence  in  fact,  except  in  so  far  as  the  quality  of  the  mother's 
blood  may  be  changed  and  may  affect  the  child.  At  no  time, 
whether  before  or  after  birth,  is  the  mother  more  than  nurse 
to  the  child.  Hereditary  influences  are  transmitted  only  through 
the  egg  cell  and  the  sperm  cell  and  these  influences  are  not  affected 
by  intra-uterine  development.  The  principles  of  heredity  and 
development  are  the  same  in  oviparous  and  in  viviparous  animals 
— in  fishes,  frogs,  birds  and  men. 

Summary. — This  is  a  very  brief  and  incomplete  statement  of 
some  of  the  important  stages  or  phases  of  the  development  of  the 
body  of  man  or  of  any  other  vertebrate.  In  all  cases  development 
begins  with  the  fertilized  egg  which  contains  none  of  the  struc- 
tures of  the  developed  animal,  though  it  may  exhibit  the  polarity 
and  symmetry  of  the  adult  and  may  also  contain  specific  kinds  of 
protoplasm  which  will  give  rise  to  specific  tissues  or  organs  of  the 
adult.  From  this  egg  cell  arise  by  division  many  cells  which  dif- 
fer from  one  another  more  and  more  as  development  proceeds, 
until  finally  the  adult  animal  results.  A  specific  type  of  develop- 
ment is  due  to  a  specific  organization  of  the  germ  cells  with  which 
development  begins,  but  the  earlier  differentiations  of  the  egg  are 
relatively  few  and  simple  as  compared  with  the  bewildering  com- 
plexities of  the  adult,  and  the  best  way  of  understanding  adult 
structures  is  to  trace  them  back  in  development  to  their  simpler 
beginnings  and  to  study  them  in  the  process  of  becoming. 

7.  Development  of  Functions. — The  development  of  functions 
goes  hand  in  hand  with  the  development  of  structures;  indeed 
function  and  structure  are  merely  different  aspects  of  one  and 
the  same  thing,  namely  organization.  All  the  general  functions 
of  living  things  are  present  in  the  germ  cells,  viz.,  (i)  Construc- 
tive and  destructive  metabolism,  (2)  Reproduction,  as  shown  in 
the  division  of  cells  and  cell  constituents,  (3)  Irritability,  or  the 
capacity  of  receiving  and  responding  to  stimuli.  All  these  gen- 
eral functions  of  living  things  are  manifested  by  germ  cells,  but 


Facts  and  Factors  of  Development  31 

as  development  advances  each  of  these  functions  becomes  more 
specialized,  more  complicated  and  more  perfect.  A  cell  which  at 
an  early  stage  was  protective,  locomotor  and  sensory  in  function 
may  give  rise  to  daughter  cells  in  which  these  functions  are  dis- 
tributed to  different  cells;  cells  which  at  an  early  stage  were 
sensitive  to  many  kinds  of  stimuli  give  rise  to  daughter  cells  which 
are  especially  sensitive  to  one  particular  kind  of  stimulus,  such  as 
vibration,  light,  or  chemicals. 

Differentiation  of  Functions  and  Structures. — Functions  de- 
velop from  a  generalized  to  a  specialized  condition  by  the  process 
of  "physiological  division  of  labor"  which  accompanies  morpho- 
logical division  of  substance.  But  just  as  in  the  union  of  hydro- 
gen and  oxygen  a  new  substance,  water,  appears  which  was  not 
present  before,  by  a  process  of  "creative  synthesis,"  and  as  in 
the  development  of  structures  new  parts  appear,  which  were  not 
present  in  the  germ,  so  new  functions  appear  in  the  course  of  de- 
velopment, which  are  not  merely  sorted  out  of  the  general  func- 
tions present  at  the  beginning,  but  which  are  created  by  the  in- 
teraction and  synthesis  of  parts  and  functions  previously  present. 
For  example,  Lane  has  shown  that  young  rats  are  quite  insensitive 
to  light  until  several  days  after  birth  although  the  eye  begins  to 
form  at  a  very  early  stage  of  development.  Doubtless  every  part 
of  the  eye  is  functioning  in  one  way  or  another  during  the  entire 
development  but  not  until  all  parts  are  formed  and  connected 
and  all  their  functions  are  synthesized  does  the  new  function, 
vision,  spring  into  existence.  Undoubtedly  the  same  is  true  of 
many  other  complex  functions  which  have  no  existence  until  all 
their  constituents  are  present  and  integrated,  when  they  sud- 
denly appear. 

Living  Functions  and  Structures  Inseparable. — Much  less  at- 
tention has  been  paid  to  the  development  of  functions  than  to  the 
development  of  structures,  and  consequently  it  is  not  possible  to 
describe  the  former  with  the  same  degree  of  detail  as  the  latter. 
But  in  spite  of  the  lack  of  detailed  knowledge  regarding  the  de- 
velopment of  particular  functions  the  general  fact  of  such  devel- 


32  Heredity  and  Environment 

opment  is  well  established.  To  what  extent  structures  may  modify 
functions  or  functions  structures,  in  the  course  of  development, 
is  a  problem  which  has  been  much  discussed,  and  upon  the  answer 
to  it  depends  the  fate  of  certain  important  theories,  for  example 
Lamarckism;  but  this  problem  can  be  solved  only  by  thorough- 
going experimental  and  analytical  work.  In  the  meantime  it  seems 
safe  to  conclude  that  living  structures  and  functions  are  insep- 
arable and  that  anything  which  modifies  one  of  these  must  of 
necessity  modify  the  other  also;  they  are  merely  different  aspects 
of  organization,  and  are  dealt  with  separately  by  the  morpholo- 
gists  and  physiologists  only  as  a  matter  of  convenience.  At  the 
same  time  there  can  be  no  doubt  that  minute  changes  of  function 
can  frequently  be  detected  where  no  corresponding  change  of 
structure  can  be  seen,  but  this  shows  only  that  physiological  tests 
may  be  more  delicate  than  morphological  ones.  In  certain  lines 
of  modern  biological  work  such  as  bacteriology,  cytology,  and  ge- 
netics, many  functional  distinctions  are  recognizable  between  or- 
ganisms that  are  morphologically  indistinguishable.  But  this  does 
not  signify  that  functional  changes  precede  structural  ones,  but 
only  that  the  latter  are  more  difficult  to  see  than  the  former.  For 
every  change  of  function  it  is  probable  that  an  "unlimited  micro- 
scopist"  could  discover  a  corresponding  change  of  structure. 

II.  DEVELOPMENT  OF  THE  MIND 

The  development  of  the  mind  parallels  that  of  the  body :  what- 
ever the  ultimate  relations  of  the  mind  and  body  may  be,  there 
can  be  no  reasonable  doubt  that  the  two  develop  together  from 
the  germ.  It  is  a  curious  fact  that  many  people  who  are  seriously 
disturbed  by  scientific  teachings  as  to  the  evolution  or  gradual  de- 
velopment of  the  human  race  accept  with  equanimity  the  universal 
observation  as  to  the  development  of  the  human  individual, — 
mind  as  well  as  body.  The  animal  ancestry  of  the  race  is  surely 
no  more  disturbing  to  philosophical  and  religious  beliefs  than  the 
germinal  origin  of  the  individual,  and  yet  the  latter  is  a  fact  of 


Facts  and  Factors  of  Development  33 

universal  observation  which  can  not  be  relegated  to  the  domain 
of  hypothesis  or  theory,  and  which  can  not  be  successfully  denied. 
If  we  admit  the  fact  of  the  development  of  the  entire  individual, 
surely  it  matters  little  to  our  philosophical  or  religious  beliefs  to 
admit  the  development  or  evolution  of  the  race. 

Ancient  Speculations. — The  origin  of  the  mind,  or  rather  of  the 
soul,  is  a  topic  upon  which  there  has  been  much  speculation  by 
philosophers  and  theologians.  One  of  the  earliest  hypotheses  was 
that  which  is  known  as  transmigration  or  metempsychosis.  This 
doctrine  probably  reached  its  greatest  development  in  ancient 
India,  where  it  formed  an  important  part  of  Buddhistic  belief ;  it 
was  also  a  part  of  the  religion  of  ancient  Egypt ;  it  was  embodied 
in  the  philosophies  of  Pythagoras  and  Plato.  According  to  these 
teachings,  the  number  of  souls  is  a  constant  one ;  souls  are  neither 
made  nor  destroyed,  but  at  birth  a  soul  which  had  once  tenanted 
another  body  enters  into  the  new  body.  This  doctrine  was  gener- 
ally repudiated  by  the  Fathers  of  the  Christian  Church.  Jerome 
and  others  adopted  the  view  that  God  creates  a  new  soul  for  each 
body  that  is  generated,  and  that  every  soul  is  thus  a  special  divine 
creation.  This  has  become  the  prevailing  view  of  the  Christian 
Church  and  is  known  as  creationism.  On  the  other  hand  Tertul- 
lian  taught  that  souls  of  children  are  generated  from  the  souls  of 
parents  as  bodies  are  from  bodies.  This  doctrine,  which  is  known 
as  traducianism,  has  been  defended  by  certain  modern  theolo- 
gians, but  has  been  formally  condemned  by  the  Roman  Catholic 
Church. 

Traducianism  undoubtedly  comes  nearer  the  scientific  teachings 
as  to  the  development  of  the  mind  than  does  either  of  the  other 
doctrines  named,  but  it  is  based  upon  the  prevalent  but  erroneous 
belief  that  the  bodies  of  the  parents  generate  the  body  of  the  child, 
and  that  correspondingly  the  souls  of  the  parents  generate  the  soul 
of  the  child.  Now  we  know  that  the  child  comes  from  germ  cells 
and  not  from  the  highly  differentiated  bodies  of  the  parents,  and 
furthermore  that  these  cells  are  not  made  by  the  parents'  bodies 
but  have  arisen  by  the  division  of  antecedent  germ  cells  (see 


34  Heredity  and  Environment 

p.  125).  Consequently  it  is  not  possible  to  hold  that  bodies  gen- 
erate bodies  or  even  germ  cells,  nor  that  souls  generate  souls. 
The  only  possible  scientific  position  is  that  the  mind  (or  soul) 
as  well  as  the  body  develops  from  the  germ. 

Certainty  of  Mental  Development. — No  fact  in  human  exper- 
ience is  more  certain  than  that  the  mind  develops  by  gradual  and 
natural  processes  from  a  simple  condition  which  can  scarcely  be 
called  mind  at  all;  no  fact  in  human  experience  is  fraught  with 
greater  practical  and  philosophical  significance  than  this,  and  yet 
no  fact  is  more  generally  disregarded.  We  know  that  the  greatest 
men  of  the  race  were  once  babies,  embryos,  germ  cells,  and  that 
the  greatest  minds  in  human  history  were  once  the  minds  of 
babies,  embryos  and  germ  cells,  and  yet  this  stupendous  fact  has 
had  but  little  influence  on  our  beliefs  as  to  the  nature  of  man  and 
of  mind.  We  rarely  think  of  Plato  and  Aristotle,  of  Shakes- 
peare and  Newton,  of  Pasteur  and  Darwin,  except  in  their  full 
epiphany,  and  yet  we  know  that  when  each  of  these  was  a  child 
he  "thought  as  a  child  and  spake  as  a  child,"  and  when  he  was  a 
germ  cell  he  behaved  as  a  germ  cell. 

Wonders  of  this  Development. — The  development  of  the  mind 
from  the  activities  of  the  germ  cells  is  certainly  most  wonderful 
and  mysterious,  but  probably  no  more  so  than  the  development  of 
the  complicated  body  of  the  adult  animal  from  the  structures  of 
the  germ.  Both  belong  to  the  same  order  of  phenomena  and 
there  is  no  more  reason  for  supposing  that  the  mind  is  supernatur- 
ally  created  than  that  the  body  is.  Indeed,  we  know  that  the  mind 
is  formed  by  a  process  of  development,  and  the  stages  of  this  de- 
velopment are  fairly  well  known.  There  is  nowhere  in  the  en- 
tire course  of  mental  development  a  sudden  appearance  of  psychi- 
cal processes,  but  rather  a  gradual  development  of  these  from 
simpler  and  simpler  beginnings.  No  detailed  study  has  been  made 
of  the  reactions  of  human  germ  cells  and  embryos,  but  there  is 
every  reason  to  believe  that  these  reactions  are  simpler  in  the 
embryo  and  germ  cell  than  in  the  infant,  and  that  they  are  gen- 


Facts  and  Factors  of  Development  35 

erally  similar  to  the  reactions  of  the  germ  cells  and  embryos  of 
other  animals,  and  to  the  behavior  of  many  lower  organisms. 

Matter  and  Mind. — A  few  years  ago  such  a  statement  would 
have  been  branded  as  "materialism"  and  promptly  rejected  with- 
out examination  by  those  who  are  frightened  by  names.  But 
the  general  spread  of  the  scientific  spirit  is  shown  not  only  by  the 
growing  regard  for  evidence  but  also  by  the  decreasing  power  of 
epithets.  "Materialism,"  like  many  another  ghost,  fades  away 
into  thin  air  or  at  least  loses  many  of  its  terrors,  when  closely 
scrutinized.  But  the  statement  that  mind  develops  from  the  germ 
cells  is  not  an  affirmation  of  materialism,  for  while  it  identifies 
the  origin  of  the  entire  individual,  mind  and  body,  with  the  devel- 
opment of  the  germ,  it  does  not  assert  that  "matter"  is  the  cause 
of  "mind"  either  in  the  germ  or  in  the  adult.  It  must  not  be  for- 
gotten that  germ  cells  are  living  things  and  that  we  go  no  further 
in  associating  the  beginnings  of  mind  with  the  beginnings  of  body 
in  the  germ  than  we  do  in  associating  mind  and  body  in  the  adult. 
It  is  just  as  materialistic  to  hold  that  the  mind  of  the  mature  man 
is  associated  with  his  body  as  it  is  to  hold  that  the  beginnings  of 
mind  in  the  germ  are  associated  with  the  beginnings  of  the  body, 
and  both  of  these  tenets  are  incontrovertible. 

Body  and  Mind. — It  seems  to  me  that  the  mind  is  related  to 
the  body  as  function  is  to  structure;  there  are  those  who  main- 
tain that  structure  is  the  cause  of  function,  that  the  real  problem 
in  evolution  or  development  is  the  transformation  of  one  struc- 
ture into  another,  and  that  the  functions  which  go  with  certain 
structures  are  merely  incidental  results;  on  the  other  hand  are 
those  who  maintain  that  function  is  the  cause  of  structure  and 
that  the  problem  of  evolution  or  development  is  the  change  which 
takes  place  in  functions  and  habits,  these  changes  causing  corre- 
sponding transformations  of  structure.  Among  adherents  of  the 
former  view  may  be  classed  many  morphologists  and  Neo-Darwin- 
ians ;  among  proponents  of  the  latter,  many  physiologists  and  Neo- 
Lamarckians.  It  seems  to  me  that  the  defenders  of  each  of  these 
views  fail  to  recognize  the  essential  unity  of  the  entire  organism, 


36  Heredity  and  Environment 

structure  as  well  as  function;  that  neither  of  these  precedes  the 
other  as  cause  precedes  effect,  though  each  may  modify  or  condi- 
tion the  other,  but  that  they  are  two  aspects  of  one  common 
thing,  viz.,  organization.  In  the  same  way  I  think  that  the  body 
or  brain  is  not  the  cause  of  mind,  nor  mind  the  cause  of  body  or 
brain,  but  that  both  are  inherent  in  one  common  organization  or 
individuality. 

In  asserting  that  the  mind  develops  from  the  germ  as  the  body 
does,  no  attempt  is  made  to  explain  the  fundamental  properties 
of  body  or  mind.  As  the  structures  of  the  body  may  be  traced 
back  to  certain  fundamental  structures  of  the  germ  cell,  so  the 
characteristics  of  the  mind  may  be  traced  back  to  certain  funda- 
mental properties  and  activities  of  the  germ.  Many  of  the 
psychical  processes  may  be  traced  back  in  their  development  to 
properties  of  sensitivity,  reflex  motions,  and  persistence  of  the 
effects  of  stimuli.  All  organisms  manifest  these  properties  and 
for  aught  we  know  to  the  contrary  they  may  be  original  and 
necessary  characteristics  of  living  things.  In  the  simplest  proto- 
plasm we  find  organization,  that  is,  structure  and  function,  and  in 
germinal  protoplasm  we  find  the  elements  of  the  mind  as  well  as 
of  the  body,  and  the  problem  of  the  ultimate  relation  of  the  two 
is  the  same  whether  we  consider  the  organism  in  its  germinal 
or  in  its  adult  stage. 

GERMINAL  BASES  OF  MIND 

In  some  way  the  mind  as  well  as  the  body  develops  out  of  the 
germ.  What  are  the  germinal  bases  of  mind  ?  What  are  the  psy- 
chical Anlagen  in  embryos  and  how  do  they  develop?  In  this 
case,  even  more  than  in  the  development  of  the  body,  we  are 
compelled  to  rely  upon  comparisons  between  human  development 
and  that  of  other  animals,  but  the  great  principle  of  the  oneness  of 
life,  as  respects  its  fundamental  processes,  has  never  yet  failed 
to  hold  true  and  will  not  fail  us  here.  In  the  study  of  the  psy- 
chical processes  of  organisms  other  than  ourselves  we  are  com- 
pelled to  rely  upon  a  study  of  their  activities,  their  reactions  to 


Facts  and  Factors  of  Development  37 

stimuli,  since  we  can  not  approach  the  subject  in  any  other  way. 
The  reactions  and  behavior  of  organisms  under  normal  and  ex- 
perimental conditions  give  the  only  insight  which  we  can  get  into 
their  psychical  processes ;  and  this  applies  to  men  no  less  than  to 
protozoa. 

i.  Sensitivity. — The  most  fundamental  phenomenon  in  the  be- 
havior of  organisms  is  irritability  or  sensitivity,  which  is  the  abil- 
ity of  receiving  and  responding  to  stimuli :  this  is  one  of  the  fun- 
damental properties  of  all  protoplasm.  But  living  matter  is  not 
equally  sensitive  to  all  stimuli,  nor  to  all  strengths  of  the  same 
stimulus.  Many  of  the  simplest  unicellular  plants  and  animals 
show  that  they  are  differentially  sensitive ;  they  often  move  toward 
weak  light  and  away  from  strong  light,  away  from  extremes  of 
heat  and  cold,  into  certain  chemical  substances  and  away  from 
others;  in  short,  all  organisms,  even  the  simplest,  may  respond 
differently  to  different  kinds  of  stimuli  or  to  different  degrees  of 
the  same  stimulus.  This  is  what  is  known  as  differential  sensitiv- 
ity (Figs.  15-19).  On  the  other  hand,  many  organisms  respond  in 
the  same  way  to  different  stimuli,  and  this  may  be  taken  to  indicate 
generally  that  they  are  not  differentially  sensitive  to  such  stimuli ; 
it  is  not  to  be  concluded  because  organisms  respond  differently  to 
certain  stimuli  that  they  are  therefore  capable  of  distinguishing 
between  all  kinds  of  stimuli,  for  this  is  certainly  not  true.  Even 
in  adult  men  the  capacity  of  distinguishing  between  different  kinds 
of  stimuli  is  far  from  perfect. 

Sensitivity  of  Germ  Cells. — Egg  cells  and  spermatozoa  show 
this  property  of  sensitivity.  The  egg  is  generally  incapable  of  lo- 
comotion, and  since  the  results  of  stimulation  must  usually  be 
detected  by  movements  it  is  not  easy  to  determine  to  what  extent 
the  egg  is  sensitive ;  but  though  the  egg  lacks  the  power  of  locomo- 
tion, it  possesses  in  a  marked  degree  the  power  of  intra-cellular 
movement  of  the  cell  contents.  When  a  spermatozoon  comes 
into  contact  with  the  surface  of  the  egg  the  cortical  protoplasm 
of  the  egg  flows  toward  that  point  and  may  form  a  cone  or  pro- 
toplasmic prominence  into  which  the  sperm  is  received  (Fig. 


Heredity  and  Environment 


4,  ec).  It  is  an  interesting  fact  that  the  same  sort  of  response 
follows  when  a  frog's  egg  is  pricked  by  a  needle,  thus  showing 
that  in  this  case  the  egg  does  not  distinguish  between  the  prick  of 
the  needle  and  that  of  the  spermatozoon.  The  spermatozoon  is 
usually  a  locomotor  cell  and  it  responds  differently  to  certain 
stimuli,  just  as  many  bacteria  and  protozoa  do;  spermatozoa  are 
strongly  stimulated  by  weak  alkali  and  alcohol,  they  gather  in 
certain  chemical  substances  and  not  in  others,  they  collect  in  great 
numbers  around  fertilizable  egg  cells,  etc. 

Sensitivity  of  Oosperm  and  of  Embryo. — The  movements  of 
fertilized  egg  cells,  cleavage  cells,  and  early  embryonic  cells  are 
usually  limited  to  flowing  movements  within  the  individual  cells. 
These  movements,  which  are  of  a  complicated  nature,  are  of  the 
greatest  significance  in  the  differentiation  of  the  egg  into  the 
embryo;  they  are  caused  chiefly  by  internal  stimuli  and  by  non- 
localized  external  ones.  Modifications  of  the  external  stimuli 
often  lead  to  modifications  of  these  intracellular  movements  and 
to  abnormal  types  of  cleavage  and  development — in  short,  these 
movements  show  that  the  fertilized  egg  is  differentially  sensitive. 

In  the  further  course  of  development  particular  portions  of  the 
embryo  become  especially  sensitive  to  some  kinds  of  stimuli,  while 
other  portions  become  sensitive  to  others.  In  this  way  the  differ- 
ent sense  organs,  each  especially  sensitive  to  one  particular  kind 
of  stimulus,  arise  from  the  generalized  sensitivity  of  the  oosperm, 
and  thus  general  sensitivity,  which  is  a  property  of  all  protoplasm, 


*B  0          1>  tf&J1  9 

FIG.  15.  DISTRIBUTION  OF  BACTERIA  IN  THE  SPECTRUM.  The  largest 
group  is  in  the  ultra-red  at  the  left;  the  next  largest  group  is  in  the  yellow-, 
orange  close  to  the  line  D.  (From  Jennings,  after  Engelmann.) 


Facts  and  Factors  of  Development 


39 


becomes  differential  sensitivity  and  special  senses  in  the  process 
of  embryonic  differentiation.  Such  sensitivity  is  the  basis  of  all 
psychic  processes ;  sensations  are  the  elements  of  the  mind. 

2.  Reflexes,  Tropisms,  Instincts— A\\  the  responses  of  germ 


FIG.  16,  a,  b,  c.  REPULSION  OF  Spirilla  BY  COMMON  SALT,  a,  condition 
immediately  after  adding  crystals ;  b  and  c,  later  stages  in  the  reaction. 

x,  y,  z,  repulsion  of  Spirilla  by  distilled  water.  The  upper  drop  consists 
of  sea-water  containing  Spirilla,  the  lower  drop,  of  distilled  water.  At  x 
these  have  just  been  united  by  a  narrow  neck;  at  y  and  z,  the  bacteria  have 
retreated  before  the  distilled  water.  (From  Jennings,  after  Massart.) 


Heredity  and  Environment 


cells,  and  of  the  simplest  organisms,  to  stimuli  are  in  the  nature 
of  reflexes  or  tropisms,  that  is,  relatively  simple,  machine-like 
responses.  "Reflex  motions"  originally  referred  to  those  re- 
sponses of  higher  animals  in  which  the  peripheral  stimuli  were 
reflected,  as  it  were,  from  the  spinal  cord  to  the  appropriate  muscles 
without  the  participation  of  the  brain.  But  at  present  the  word 
"reflex"  has  come  to  have  a  much  broader  application  and  is  used 
for  all  simple,  automatic  responses,  even  though  there  are  no 
nerves  and  even  when  the  response  is  not  movement  but  secretion, 
metabolism  or  any  other  activity.  "Tropisms,"  on  the  other  hand, 
is  a  more  specific  term  and  refers  to  the  movements  of  organisms 

19 


19* 


38  Q 


10*  25* 

FIG.  17.  REACTIONS  OF  Paramecium  TO  HEAT  AND  COLD.  At  a  the  in- 
fusoria are  uniformly  distributed  in  a  trough,  both  ends  of  which  have  a 
temperature  of  19° ;  at  b  the  infusoria  are  shown  collected  at  the  cooler 
end  of  the  trough;  at  c  they  have  collected  at  the  warmer  end  of  the 
trough.  (From  Jennings,  after  Mendelssohn.) 


Facts  and  Factors  of  Development  41 

toward  or  away  from  a  source  of  stimulus,  the  former  being 
known  as  positive  and  the  latter  as  negative^  tropism.  When  re- 
sponses are  very  complex,  one  response  calling  forth  another  and 
involving  many  complex  reflexes  or  chains  of  reflexes,  as  is  fre- 


FIG.  18.  PHOTOTROPISM  OF  SEEDLING  OF  WHITE  MUSTARD  supported  by  a 
sheet  of  cork  (K,  K}  floating  on  the  water.  The  direction  from  which 
light  comes  is  shown  by  the  arrows ;  the  stem  and  leaves  are  turned  toward 
the  light  (positive  phototropism),  the  root  away  from  the  light  (negative 
phototropism).  (After  Strasburger.) 


42  Heredity  and  Environment 

quently  the  case  in  animals,  they  are  known  as  "instincts."  Re- 
flexes and  tropisms  occur  in  the  simplest  organisms,  such  as  bac- 
teria, protozoa,  ancf  single  cells,  as  well  as  in  higher  plants  and 
animals  (Figs.  15-19),  but  instincts  are  limited  to  animals  with 
a  nervous  system. 

Reflexes  and  Tropisms  of  Germ  Cells  and  Embryos  are  seen  in 
movements  of  spermatozoa,  movements  of  the  protoplasm  in  egg 
cells  and  embryonic  cells,  movements  of  cells  and  cell  masses  in 
the  formation  of  the  gastrula,  alimentary  canal,  nervous  system 
and  other  organs.  Indeed  the  entire  process  of  development, 
whether  accompanied  by  visible  movements  or  not,  may  be  re- 
garded as  a  series  of  automatic  responses  to  stimuli.  When  the 
embryo  becomes  differentiated  to  such  an  extent  as  to  have  spe- 
cialized organs  for  producing  movement  its  capacity  for  making 
responsive  movements  to  stimuli  becomes  much  increased.  In  the 
embryo  the  rhythmic  contractions  of  heart,  anmion  and  intestine 
are  early  manifestations  of  reflex  motions.  These  appear  chiefly 
in  the  involuntary  muscles  before  nervous  connections  are 


FIG.  19.  GEOTROPISM  OF  SEEDLING  OAK.  After  starting  to  grow  with  the 
axis  A-A  in  vertical  position  the  seedling  was  gradually  turned  through 
90°  until  the  axis  B-B  was  vertical;  during  this  change  of  position  the  stem 
continues  to  grow  upward  (negative  geotropism),  the  root  downward 
(positive  geotropism)  as  indicated  by  dotted  outlines. 


Facts  and  Factors  of  Development  43 

formed,  the  protoplasm  of  the  muscle  cells  probably  responding 
directly  to  the  chemical  stimulus  of  certain  salts  in  the  body  fluids, 
as  Loeb  has  shown  in  other  cases.  Reflexes  which  appear  later 
are  the  "random  movements"  of  the  voluntary  muscles  of  limbs 
and  body,  which  are  called  forth  by  nerve  impulses.  Tropisms  are 
manifested  only  by  organisms  capable  of  considerable  free  move- 
ment and  hence  are  absent  in  the  foetus  though  present  in  many 
free-living  larvae. 

Development  of  Instincts. — Some  instincts  are  present  imme- 
diately after  birth,  such  as  the  instinct  of  sucking  or  crying  in  the 
human  infant,  though  these  are  so  simple  when  compared  with 
some  instincts  which  develop  later  that  they  might  be  classed  as 
reflexes ;  it  is  doubtful  whether  any  of  the  activities  before  birth 
could  properly  be  designated  as  instincts.  Reflexes,  tropisms  and 
instincts  have  had  a  phylogenetic  as  well  as  an  ontogenetic  origin, 
and  consequently  we  might  expect  that  they  would  in  general  make 
for  the  preservation  of  the  species ;  as  a  matter  of  fact  we  usually 
find  that  they  are  remarkably  adapted  to  this  end.  For  instance 
the  instincts  of  the  human  infant  to  grasp  objects,  to  suck  things 
which  it  can  get  into  its  mouth,  to  cry  when  in  pain,  are  compli- 
cated reflexes  which  have  survived  in  the  course  of  evolution, 
probably  becauSfe  they  serve  a  useful  purpose. 

Very  much  has  been  written  on  the  nature  and  origin  of  in- 
stincts, but  the  best  available  evidence  strongly  favors  the  view 
that  instincts  are  complex  reflexes,  which,  like  the  structures  of 
an  organism,  have  been  built  up,  both  ontogenetically  and  phylo- 
genetically,  under  the  stress  of  the  elimination  of  the  unfit,  so  that 
they  are  usually  adaptive. 

3.  Summation  of  Stimuli,  Memory, — Another  general  character- 
istic of  protoplasm  is  the  capacity  of  storing  up  or  registering  the 
effects  of  previous  stimuli.  A  single  stimulus  may  produce 
changes  in  an  organism  which  persist  for  a  longer  or  shorter 
time,  and  if  a  second  stimulus  occurs  while  the  effect  of  a  previous 
stimulus  still  persists,  the  response  to  the  second  stimulus  may  be 


44 


Heredity  and  Environment 


very  different  from  that  to  the  first.  Macfarlane  found  that  if  the 
sensitive  hairs  on  the  leaf  of  Dionaea,  the  Venus  fly-trap  (Fig.  20, 
SH),  be  stroked  once  no  visible  response  is  called  forth,  but  if 
they  be  stroked  a  second  time  within  three  minutes  the  leaf  in- 
stantly closes.  If  a  longer  period  than  three  minutes  elapses  after 
the  first  stimulus  and  before  the  second  no  visible  response  fol- 
lows, i.e.,  two  successive  stimuli  are  necessary  to  cause  the 
leaves  to  close,  and  the  two  must  not  be  more  than  three  minutes 
apart;  the  effects  of  the  first  stimulus  are  in  some  way  stored  or 
registered  in  the  leaf  for  this  brief  time.  This  kind  of  phenome- 
non is  widespread  among  living  things  and  is  known  as  "summa- 
tion of  stimuli."  In  all  such  cases  the  effects  of  a  former  stimu- 
lus are  in  some  way  stored  up  for  a  longer  or  shorter  time  in  the 
protoplasm.  It  is  possible  that  this  is  the  result  of  the  formation 


FIG.  20.  Dionaea  muscipula  (VENUS'  FLY-TRAP).  Three  leaves  showing 
marginal  teeth  and  sensitive  hairs  (SH).  The  leaf  at  the  left  is  fully 
expanded,  the  one  at  the  right  is  closed. 


Facts  and  Factors  of  Development  45 

of  some  chemical  substance  which  remains  in  the  protoplasm  for 
a  certain  time,  during  which  iime  the  effects  of  the  stimulus  are 
said  to  persist,  or  it  may  be  due  to  some  physical  change  in  the 
protoplasm  analogous  to  the  "set"  in  metals  which  have  been 
subjected  to  mechanical  strain. 

Organic  Memory. — Probably  of  a  similar  character  is  the  per- 
sistence of  the  effects  of  repeated  stimuli  and  responses  on  any 
organ  of  a  higher  animal.  A  muscle  which  has  contracted  many 
times  in  a  definite  way  ultimately  becomes  "trained"  so  that  it 
responds  more  rapidly  and  more  accurately  than  an  untrained 
muscle;  and  the  nervous  mechanism  through  which  the  stimulus 
is  transmitted  also  becomes  trained  in  the  same  way.  Indeed  such 
training  is  probably  chiefly  a  training  of  the  nervous  mechanism. 
The  skill  of  the  pianist,  of  the  tennis  player,  of  the  person  who  has 
learned  the  difficult  art  of  standing  and  walking,  or  the  still  more 
difficult  art  of  talking,  is  probably  due  to  the  persistence  in  mus- 
cles and  nerves  of  the  effects  of  many  previous  activities.  All 
such  phenomena  were  called  by  Hering  "organic  memory,"  to  in- 
dicate that  this  persistence  of  the  effects  of  previous  activities  in 
muscles  and  other  organs  is  akin  to  that  persistence  of  the  effects 
of  previous  experiences  in  the  nervous  mechanism  which  we  com- 
monly call  memory. 

Associative  Memory. — It  seems  probable  that  this  ability  of  pro- 
toplasm in  general  to  preserve  for  a  time  the  effects  of  former 
stimuli  is  fundamentally  of  the  same  nature  as  the  much  greater 
power  of  nerve  cells  to  preserve  such  effects  for  much  longer 
periods  and  in  complex  associations,  a  faculty  which  is  known 
as  associative  memory.  The  embryos,  and  indeed  even  the  germ 
cells  of  higher  animals,  may  safely  be  assumed  to  be  endowed 
with  protoplasmic  and  organic  memory,  out  of  which,  in  all 
probability,  develops  associative  and  conscious  memory  in  the  ma- 
ture organism. 

4.  Intellect,  Reason. — Even  the  intellect  and  reason  which  so 
strongly  characterize  man  have  had  a  development  from  rela- 


46  Heredity  and  Environment 

tively  simple  beginnings.  All  children  come  gradually  to  an  age  of 
intelligence  and  reason.  In  its  simpler  forms  at  least  intelligence 
is  the  capacity  of  consciously  profiting  by  experience,  while  reas- 
oning consists  in  the  comparison  of  past  experiences  with  new 
and  more  or  less  different  phenomena.  In  the  absente  of  indi- 
vidual experience  young  children  have  none  o*f  this  power,  but 
it  comes  gradually  as  a  result  of  remembering  past  experiences 
and  of  fitting  such  experiences  into  new  conditions. 

Useful  Responses.— Young  infants  and  many  lower  animals 
lack  intelligence  or  reason,  though  their  behavior  is  frequently  of 
such  a  sort  as  to  suggest  that  they  are  reasoning.  Even  the  low- 
est animals  avoid  injurious  substances  and  conditions  and  find 
beneficial  ones;  more  complex  animals  learn  to  move  objects,  solve 
problems,  and  find  their  way  through  labyrinths  in  the  shortest 
and  most  economical  way ;  but  this  apparently  intelligent  and  pur- 
posive behavior  has  been  shown  to  be  due  to  the  gradual  elimina- 
tion of  all  sorts  of  useless  activities,  and  to  the  persistence  of  the 
useful  ones. 

The  ciliated  infusorian,  Paramecium,  moves  by  the  beating  of 
cilia,  which  are  arranged  in  such  a  way  that  they  drive  the  ani- 
mal forward  in  a  spiral  course.  However,  when  it  is  strongly  ir- 
ritated, the  normal  forward  movement  is  reversed;  the  cilia  beat 
forward  instead  of  backward  and  the  animal  is  driven  backward 
for  some  distance  (Fig.  21,  I,  2,  3)  ;  it  then  stands  nearly  still, 
merely  rolling  over  and  swerving  toward  the  aboral  side,  and 
finally  it  goes  ahead  again,  usually  on  a  new  course  (Fig.  21, 
3,  4,  5,  6).  These  movements  seem  to  be  conditioned  rather 
rigidly  by  the  organization  of  the  animal:  they  are  more  or  less 
fixed  and  mechanical  in  character,  though  to  a  certain  extent 
they  may  be  modified  by  experience  or  physiological  states.  Para- 
mecium behaves  as  it  does  by  virtue  of  its  constitution,  just  as  an 
egg  develops  in  a  particular  way  because  of  its  particular  organi- 
zation. 

"Trial  and  Error" — But  although  limited  in  its  behavior  to 


Facts  and  Factors  of  Development  47 

these  relatively  simple  motor  reactions,  Paramecium  does  many 
things  which  seem  to  show  intelligence  and  purpose.  It  avoids 
many  injurious  substances,  such  as  strong  salts  or  acids,  and  it 
collects  in  non-injurious  or  beneficial  substances,  such  as  weak 
acids,  masses  of  bacteria  upon  which  it  feeds,  etc.  It  avoids  ex- 
tremes of  heat  and  cold  and  if  one  end  of  a  dish  containing  Para- 
mecla  is  heated  and  the  other  end  is  cooled  by  ice,  the  Paramecia 
collect  in  the  region  somewhere  between  these  two  extremes 
(Fig.  17).  Jennings,  by  studying  carefully  the  behavior  of  sin- 
gle individuals,  established  the  fact  that  this  apparently  intelli- 
gent action  is  due  to  differential  sensitivity  and  to  the  single  motor 
reaction  of  the  animal.  If  in  the  course  of  its  swimming  a  Para- 
mecium  comes  into  contact  with  an  irritating  substance  or  con- 
dition, it  backs  a  short  distance,  swerves  toward  its  aboral  side, 
and  goes  ahead  in  a  new  path ;  if  it  again  comes  in  contact  with 
the  irritating  conditions  this  reaction  is  repeated,  and  so  on  in- 
definitely until  finally  a  path  is  found  in  which  the  cause  of  irri- 
tation is  avoided  altogether.  In  short,  Parameciutn  continually 
tries  its  environment,  and  backs  away  from  irritating  substances 
or  conditions.  Its  apparently  intelligent  reactions  are  thus  ex- 
plained as  due  to  a  process  of  "trial  and  error."* 

The  behavior  of  worms,  star-fishes,  crustaceans,  mollusks,  as 
well  as  of  fishes,  frogs,  reptiles,  birds  and  mammals,  has  been 
studied  and  in  all  cases  it  is  found  that  their  method  of  responding 
to  stimuli  is  not  at  first  really  purposive  and  intelligent  but  by  the 

*  In  Paramecium,  there  is  certainly  no  consciousness  of  trial  and  error, 
and  probably  no  unconscious  attempt  on  the  part  of  the  animal  to  attain 
certain  ends.  Its  responses  are  reflexes  or  tropisms,  which  are  determined 
by  the  nature  of  the  animal  and  the  character  of  the  stimulus.  The  fact 
that  these  responses  are  in  the  main  self-preservative  is  due  to  the  teleo- 
logical  organization  of  Paramecium  which  has  been  evolved,  according  to 
current  opinion,  as  the  result  of  long  ages  of  the  elimination  of  the  unfit. 
If,  in  the  opinion  of  any  one,  the  expression  "trial  and  error"  necessarily 
involves  a  striving  after  ends,  it  would  be  advisable  to  replace  it  in  this 
case  by  some  such  term  as  "useful  or  adaptive  reactions." 


48 


Heredity  and  Environment 


gradual  elimination  of  useless  responses  and  the  preservation  (or 
remembering)  of  useful  ones  the  behavior  may  come  to  be  pur- 
posive and  intelligent. 

Intelligence  Develops  from  Trial  and  Error. — Thorndike  found 
that  when  dogs,  cats  or  monkeys  were  confined  in  cages  which 
could  be  opened  from  the  inside  by  turning  a  button,  or  pressing 
upon  a  lever,  or  pulling  a  cord,  they  at  first  clawed  around  all 
sides  of  the  cage  until  by  chance  they  happened  to  operate  the 
mechanism  which  opened  the  door.  Thereafter  they  gradually 
learned  by  experience,  that  is,  by  trial  and  error,  and  finally  by 
trial  and  success,  just  where  and  how  to  claw  in  order  to  get  out 
at  once.  When  a  dog  has  learned  to  turn  a  button  at  once  and 
open  a  door  we  say  he  is  intelligent,  and  if  he  can  learn  to  apply 
his  knowledge  of  any  particular  cage  to  other  and  different  cages, 
a  thing  which  Thorndike  denies,  we  should  be  justified  in  saying 
that  he  reasons,  though  in  this  case  intelligence  and  reason  are 
founded  upon  memory  of  many  past  experiences,  of  many  trials 
and  errors  and  of  a  few  trials  and  successes. 


FIG.  21.  DIAGRAM  OF  THE  AVOIDING  REACTION  OF  Paramecium.  A  is  a 
solid  object  or  other  source  of  stimulation.  1-6,  successive  positions  oc- 
cupied by  the  animal.  The  rotation  on  the  long  axis  is  not  shown.  (After 
Jennings.) 


Facts  and  Factors  of  Development  49 

There  is  every  evidence  that  human  beings  arrive  at  intelli- 
gence and  reason  by  the  same  process,  a  process  of  many  trials 
and  errors  and  a  few  trials  and  successes,  a  remembering  of  these 
past  experiences  and  an  application  of  them  to  new  conditions.  A 
baby  grasps  for  things  which  are  out  of  its  reach,  until  it  has 
learned  by  experience  to  appreciate  distances ;  it  tests  all  sorts  of 
pleasant  and  unpleasant  things  until  it  has  learned  to  avoid  the 
latter  and  seek  the  former ;  it  experiments  with  its  own  body  until 
it  has  learned  what  it  can  do  and  what  it  can  not  do.  Is  not  this 
learning  by  experience  akin  to  the  same  process  in  the  dog  and 
more  remotely  to  the  trial  and  error  of  the  earthworm  or  the 
adaptive  reflexes  of  Parameciuni  ?  Is  not  intelligence  and  reason 
in  all  of  us,  and  upon  all  subjects,  based  upon  the  same  processes 
of  trial  and  error,  memory  of  past  experiences  and  application  of 
these  to  new  conditions?  Surely  this  is  true  in  all  experimental 
and  scientific  work.  Indeed  the  scientific  method  is  the  method  of 
trial  and  error,  and  finally  trial  and  success — the  method  recom- 
mended by  St.  Paul  to  'prove  all  things  and  hold  fast  that  which 
is  good/ 

Learning  by  Experience. — In  Paramecium  the  reflex  type  of 
behavior  is  relatively  complete;  there  is  no  associative  memory 
and  no  ability  to  learn  by  experience.  In  the  earthworm  associa- 
tive memory  is  but  slightly  developed  and  the  animal  learns  but 
little  by  experience  and  can  make  no  application  of  past  ex- 
periences to  new  conditions.  In  the  dog  associative  memory  is 
well  developed;  the  animal  learns  by  experience  and  can,  to  a 
limited  extent,  apply  such  memory  of  past  experiences  to  new 
conditions.  In  adult  man  all  of  these  processes  are  fully  devel- 
oped and  particularly  the  last,  viz.,  the  ability  to  reason.  But  in 
his  development  the  human  individual  passes  through  the  more 
primitive  stages  of  intelligence,  represented  by  the  lower  animals 
named;  the  germ  cells  and  embryo  represent  only  the  stages  of 
reflex  behavior,  to  these  trial  and  error  and  associative  memory 
are  added  in  the  infant  and  young  child,  and  to  these  the  appli- 


50  Heredity  and  Environment 

cation  of  past  experience  to  new  conditions,  or  reason,  is  added  in 
later  years. 

5.  Will. — Another  characteristic,  which  many  persons  regard 
as  the  supreme  psychical  faculty,  is  the  will.  This  faculty  also 
undergoes  development  and  from  relatively  simple  beginnings. 
The  will  of  the  child  has  developed  out  of  something  which  is  far 
less  perfect  in  the  infant  and  embryo  than  in  the  child.  Obser- 
vations and  experiments  on  lower  animals  and  on  human  beings, 
as  well  as  introspective  study  of  our  own  activities,  appear  to 
justify  the  following  conclusions: 

(i.)  No  Activity  Without  Stimuli. — Every  activity  of  an  organ- 
ism is  a  response  to  one  or  more  stimuli,  external  or  internal  in 
origin.  These  stimuli  are  in  the  main,  if  not  entirely,  energy 
changes  outside  or  inside  the  organism.  In  lower  organisms  as 
well  as  in  the  germ  cells  and  embryos  of  higher  animals  the  pos- 
sible number  of  responses  are  few  and  prescribed  owing  to  their 
relative  simplicity,  and  the  response  follows  the  stimulus  direct- 
ly. In  more  complex  organisms  the  number  of  possible  responses 
to  a  stimulus  is  greatly  increased,  and  the  visible  response  may  be 
the  end  of  a  long  series  of  internal  changes  which  are  started  by 
the  original  stimulus. 

(2.)  Inhibitions. — The  response  to  a  stimulus  may  be  modified 
or  inhibited  in  the  following  ways: 

(a)  Conflicting  Stimuli. — Through  conflicting  stimuli  and 
changed  physiological  states,  due  to  fatigue,  hunger,  etc.  Many 
stimuli  may  reach  the  organism  at  the  same  time  and  if  they  con- 
flict they  may  nullify  one  another  or  the  organism  may  respond 
to  the  strongest  stimulus  and  disregard  the  weaker  ones.  When 
an  organism  has  begun  to  respond  to  one  stimulus  it  is  not  easily 
diverted  to  another.  Jennings  found  that  the  attached  infusor- 
ian,  Stentor,  which  usually  responds  to  strong  stimuli  by  closing 
up,  may,  when  repeatedly  stimulated,  loosen  its  attachment  and 
swim  away,  thus  responding  in  a  wholly  new  manner  when  its 
physiological  state  has  been  changed  by  repeated  stimuli  and  re- 


Facts  and  Factors  of  Development  51 

sponses.  Whitman  found  that  leeches  of  the  genus  Clepsine 
prefer  shade  to  bright  light,  and  other  things  being  equal  they 
always  seek  the  under  sides  of  stones  and  shaded  places;  but  if 
a  turtle  from  which  they  normally  suck  blood  is  put  into  an  aqua- 
rium with  leeches,  they  at  once  leave  the  shade  and  attach  them- 
selves to  the  turtle.  They  prefer  shade  to  bright  light  but  they 
prefer  their  food  to  the  shade.  The  tendency  to  remain  concealed 
is  inhibited  by  the  stronger  stimulus  of  hunger.  On  the  other 
hand  he  found  that  the  salamander,  Necturus,  is  so  timid  that  it 
will  not  take  food,  even  though  starving,  until  by  gradual  stages 
and  gentle  treatment  its  timidity  can  be  overcome  to  a  certain 
extent.  Here  fear  is  at  first  a  stronger  stimulus  than  hunger 
and  unless  the  stimulus  of  fear  can  be  reduced  the  animal  will 
starve  to  death  in  the  presence  of  the  most  tempting  food. 

(b)  Compulsory  Limitations. — Responses  may  also  be  modi- 
fied through  compulsory  limitation  of  many  possible  responses  to 
a  particular  one,  and  the  consequent  formation  of  a  habit.  This 
is  the  method  of  education  employed  in  training  all  sorts  of  ani- 
mals. Thus  Jennings  found  that  a  star-fish  could  be  trained  to 
turn  itself  over,  when  placed  on  its  back,  by  means  of  one  pair 
of  arms  simply  by  persistently  preventing  the  use  of  the  other 
arms.  Many  responses  of  organisms  are  modified  in  a  similar 
way,  not  only  by  artificial  limitations  but  also  by  natural  ones. 

(3)  Fixed  and  Plastic  Behavior. — Responses  which  have  be- 
come fixed  and  constant  through  natural  selection  or  other  means 
of  limitation  may  become  more  varied  and  general  when  the  com- 
pulsory limitation  is  relaxed.  Behavior  in  the  former  case  is  fixed 
and  instinctive,  in  the  latter  more  varied  and  plastic.  Thus  Whit- 
man found  that  the  behavior  of  domesticated  pigeons  is  more  var- 
iable and  their  instincts  are  less  rigidly  fixed  than  in  wild  species. 
If  the  eggs  of  the  wild  passenger  pigeon  are  removed  to  a  little 
distance  from  the  nest  the  pigeon  returns  to  the  nest  and  sits  down 
as  if  nothing  had  happened.  She  soon  finds  out,  not  by  sight  but 
by  feeling,  that  something  is  missing,  and  she  leaves  the  nest  after 


52  Heredity  and  Environment 

a  few  minutes  without  heeding  the  eggs.  The  ring-neck  pigeon 
also  misses  the  eggs  and  sometimes  rolls  one  of  them  back  into 
the  nest,  but  never  attempts  to  recover  more  than  one.  The  dove- 
cote pigeon  generally  tries  to  recover  both  eggs.  According  to 
Whitman : 

In  these  three  grades  the  advance  is  from  extreme  blind  uni- 
formity of  action,  with  little  or  no  choice,  to  a  stage  of  less  rigid 
uniformity  ....  Under  conditions  of  domestication  the  action  of 
natural  selection  has  been  relaxed,  with  the  result  that  the  rigor  of 
instinctive  co-ordination,  which  bars  alternative  action,  is  more  or 
less  reduced.  Not  only  is  the  door  to  choice  thus  unlocked,  but 
more  varied  opportunities  and  provocations  arise,  and  thus  the  in- 
ternal mechanism  and  the  external  conditions  and  stimuli  work 
both  in  the  same  direction  to  favor  greater  freedom  of  action. 
When  choice  thus  enters  no  new  'factor  is  introduced.  There  is 
greater  plasticity  within  and  more  provocation  without,  and  hence 
the  same  bird,  without  the  addition  or  loss  of  a  single  nerve  cell, 
becomes  capable  of  higher  action  and  is  encouraged  and  even 
constrained  by  circumstances  to  learn  to  use  its  privileges  of 
choice.  Choice,  as  I  conceive  it,  is  not  introduced  as  a  little  deity 
encapsuled  in  the  brain.  .  .  .  But  increased  plasticity  invites  great- 
er interaction  of  stimuli  and  gives  more  even  chances  for  con- 
flicting impulses. 

(4)  Conscious  Choice  or  Will. — Finally  in  all  animals  behavior 
is  modified  through  previous  experience,  just  as  structure  is  also. 
Where  several  responses  to  a  stimulus  are  possible  and  where 
experience  has  taught  that  one  response  is  more  satisfactory  than 
another,  action  may  be  limited  to  this  particular  response,  not  by 
external  compulsion  but  by  the  internal  impulse  of  experience 
and  intelligence.  This  is  what  we  know  as  conscious  choice  or 
will.  Whitman  says: 

Choice  runs  on  blindly  at  first  and  ceases  to  be  blind  only  in 
proportion  as  the  animal  learns  through  nature's  system  of  com- 
pulsory education.  The  teleological  alternatives  are  organically 
provided;  one  is  taken  and  fails  to  give  satisfaction,  another  is 
tried  and  gives  contentment.  This  little  freedom  is  the  dawning 


Facts  and  Factors  of  Development  53 

grace  of  a  new  dispensation,  in  which  education  by  experience 
comes  in  as  an  amelioration  of  the  law  of  elimination.  .  .  .  Intelli- 
gence implies  varying  degrees  of  freedom  of  choice,  but  never 
complete  emancipation  from  automatism. 

Freedom  of  action  does  not  mean  action  without  stimuli,  but 
rather  the  introduction  of  the  results  of  experience  and  intelli- 
gence as  additional  stimuli.  The  activities  which  in  lower  animals 
are  "cabined,  cribbed,  confined,"  reach  in  man  their  fullest  and 
freest  expression ;  but  the  enormous  difference  between  the  rela- 
tively fixed  behavior  of  a  protozoan  or  a  germ  cell  and  the  rela- 
tively free  activity  of  a  mature  man  is  bridged  not  only  in  the 
process  of  evolution,  but  also  in  the  course  of  individual  develop- 
ment. 

6.  Consciousness. — The  most  complex  of  all  psychic  phenomena, 
indeed  the  one  which  includes  many  if  not  all  of  the  others,  is 
consciousness.  Like  every  other  psychic  process  this  has  under- 
gone development  in  each  of  us ;  we  not  only  came  out  of  a  state 
of  unconsciousness,  but  through  several  years  we  were  gradually 
acquiring  consciousness  by  a  process  of  development.  Whether 
consciousness  is  the  sum  of  all  the  psychic  faculties,  or  is  a  new 
product  dependent  upon  the  interaction  of  the  other  faculties, 
it  must  pass  through  many  stages  in  the  course  of  its  development, 
stages  which  would  commonly  be  counted  as  unconscious  or  sub- 
conscious states,  and  complete  consciousness  must  depend  upon 
the  complete  development  and  activity  of  the  other  faculties,  par- 
ticularly associative  memory  and  intelligence. 

Germ  Cells  Not  Conscious. — The  question  is  sometimes  asked 
whether  germ  cells,  and  indeed  all  living  things,  may  not  be  con- 
scious in  some  vague  manner.  One  might  as  well  ask  whether 
water  is  present  in  hydrogen  and  oxygen.  Doubtless  the  elements 
out  of  which  consciousness  develops  are  present  in  the  germ  cells, 
in  the  same  sense  that  the  elements  of  the  other  psychic  processes 
or  of  the  organs  of  the  body  are  there  present ;  not  as  a  miniature 
of  the  adult  condition,  but  rather  in  the  form  of  elements  or  fac- 


54  Heredity  and  Environment 

tors,  which  by  a  long  series  of  combinations  and  transformations, 
due  to  interactions  with  one  another  and  with  the  environment, 
give  rise  to  the  fully  developed  condition. 

Continuity  of  Consciousness. — Finally  there  seems  good  rea- 
son for  believing  that  the  continuity  of  consciousness,  the  con- 
tinuing sense  of  identity,  is  associated  with  the  continuity  of  or- 
ganization, for  in  spite  of  frequent  changes  of  the  materials  of 
which  we  are  composed  our  sense  of  identity  remains  undis- 
turbed. However,  the  continuity  of  protoplasmic  and  cellular 
organization  generally  remains  undisturbed  throughout  life,  and 
the  continuity  of  consciousness  is  associated  with  this  continuity 
of  organization,  especially  in  certain  parts  of  the  brain.  It  is  an 
interesting  fact  that  in  man,  and  in  several  other  animals  which 
may  be  assumed  to  have  a  sense  of  identity,  .the  nerve  cells,  espe- 
cially those  of  the  brain,  cease  dividing  at  an  early  age,  and  these 
identical  cells  persist  throughout  the  remainder  of  life.  If  nerve 
cells  continued  to  divide  throughout  life,  as  epithelial  cells  do, 
there  would  be  no  such  persistence  of  identical  cells,  and  one  is 
free  to  speculate  that  in  such  cases  there  would  be  no  persistence 
of  the  sense  of  identity. 

Organization  includes  both  structure  and  function,  and  con- 
tinuity of  organization  implies  not  only  persistence  of  protoplas- 
mic and  cellular  structures  but  also  persistence  of  the  functions 
of  sensitivity,  reflexes,  memory,  instincts,  intelligence,  and  will ; 
the  continuity  of  consciousness  is  associated  with  the  continuity 
of  these  activities  as  well  as  with  the  structures  of -the  body  in  gen- 
eral and  of  the  brain  in  particular.  It  is  well  known  that  things 
which  interrupt  or  destroy  these  functions  or  structures  interrupt 
or  destroy  consciousness.  Lack  of  oxygen,  anesthetics,  normal 
sleep  cause  in  some  way  a  temporary  interruption  of  these  func- 
tions and  consequently  temporary  loss  of  consciousness;  while 
certain  injuries  or  diseases  of  the  brain  which  bring  about  the 
destruction  of  certain  centers  or  association  tracts  may  cause 
permanent  loss  of  consciousness. 


Facts  and  Factors  of  Development  55 

7.  Parallel  Development  of  Body  and  Mind. — The  development 
of  all  of  these  psychical  faculties  runs  parallel  with  the  develop- 
ment of  bodily  structures  and  apparently  the  method  of  develop- 
ment in  the  two  cases  is  similar,  viz.,  progressive  differentiation 
of  complex  and  specialized  structures  and  functions  from  relative- 
ly simple  and  generalized  beginnings.  Indeed  the  entire  organism, 
structure  and  function,  body  and  mind,  is  a  unity,  and  the  only 
justification  for  dealing  with  these  constituents  of  the  organism 
as  if  they  were  separate  entities,  whether  they  be  regarded  in 
their  adult  condition  or  in  the  course  of  their  development,  is  to 
be  found  in  the  increased  convenience  and  effectiveness  of  such 
separate  treatment. 

Development,  like  many  other  vital  phenomena,  may  be  con- 
sidered from  several  different  points  of  view,  such  as  (i)  physico- 
chemical  events  involved,  (2)  physiological  processes,  (3)  mor- 
phological features,  (4)  ecological  correlations  and  adaptations, 
(5)  psychological  phenomena,  (6)  social  and  moral  characteris- 
tics. All  of  these  phases  of  development  are  correlated;  indeed 
they  are  parts  of  one  general  process,  and  a  complete  account  of 
this  process  must  include  them  all.  General  considerations  may 
lead  us  to  the  belief  that  each  of  the  succeeding  aspects  of  devel- 
opment named  above  may  be  causally  explained  in  terms  of  the 
preceding  ones,  and  hence  all  be  reducible  to  physics  and  chem- 
istry. But  this  is  not  now  demonstrable  and  may  not  be  true. 
Function  and  structure  may  be  related  causally,  or  they  may  be 
two  aspects  of  one  substance.  The  same  is  true  of  body  and  mind 
or  of  matter  and  energy.  But  even  if  each  of  these  different 
phases  in  the  development  of  personality  may  not  be  causally  ex- 
plained by  the  preceding  ones,  at  least  the  principle  of  explanation 
employed  for  any  aspect  of  development  ought  to  be  consistent 
and  harmonious  with  that  employed  for  any  other  aspect. 

The  phenomena  of  mental  development  in  man  and  other  ani- 
mals may  be  summarized  as  follows: 


Heredity  and  Environment 


DEVELOPMENT  OF  PSYCHICAL  PROCESSES  IN  ONTOGENY  AND  PHYLOGENY  OUT 

OF  GENERAL  IRRITABILITY  OR  SENSITIVITY — CAPACITY 

OF  RESPONDING  TO  STIMULI 


ALL  LIVING  THINGS,  INCLUDING 
GERM  CELLS  AND  EMBRYOS, 
SHOW: 

1.  Differential  Sensitivity  = 
Different    Responses   to    Stimuli 

differing  in  Kind  or  Quantity. 

2.  Reflexes,  Tropisms  = 
Relatively     Simple,     Mechanical 

Responses. 

3.  Organic  Memory  = 

Results  of  Previous  Experience 
registered  in  General  Proto- 
plasm. 

4.  Adaptive  Responses  = 
Results  of  Elimination  of  Useless 

Responses   through   Trial    and 
Error. 

5.  Varied  Responses  = 
Dependent  upon  Conflicting  Stim- 
uli and  Physiological   States. 

6.  Identity  = 

Continuity  of  Individual  Organi- 
zation. 

7.  Subjective  Phenomena,  if  any, 
Accompanying      preceding      pro- 
cesses. 


MATURE   FORMS   OF   HIGHER  ANI- 
MALS SHOW: 

i.  Special  Senses  and  Sensations  = 
Differentiated  out  of  General  Senses 

and  Sensations. 
2.  Instincts      (Inherited),     Habits 

(Acquired)  = 
Complex      Reflexes,       involving 

Nerve  Centers. 
3  Associative  Memory  •= 
Results  of  Experience  registered 
in  Nerve  Centers  and  Associa- 
tion Tracts. 

4.  Intelligence,  Reason  = 
Results  of  Trial  and  Error  plus 

Associative  Memory,  i.e.,  Ex- 
perience. 

5.  Inhibition,  Choice,  Will  = 
Dependent       upon      Associative 

Memory,   Intelligence,   Reason. 

6.  Consciousness  = 

Continuity    of    Memory,    Intelli- 
gence, Reason,  Will. 

7.  Feelings,  Emotions  — 
Accompanying  one  or  more  of  pre- 
ceding processes. 


B.     FACTORS    OF    DEVELOPMENT 

These  are  some  of  the  facts  of  development, — a  very  incom- 
plete resume  of  some  of  the  stages  through  which  a  human  being 
passes  in  the  course  of  his  development  from  the  germ.  What  are 
the  factors  of  development  ?  *  By  what  processes  is  it  possible  to 
derive  from  a  relatively  simple  germ  cell  the  complexities  of  an 
adult  animal?  How  can  mind  and  consciousness  develop  out  of 


Facts  and  Factors  of  Development  57 

the  relatively  simple  psychical  elements  of  the  germ?  These 
are  some  of  the  great  problems  of  development — one  of  the 
greatest  and  most  far-reaching  themes  which  has  ever  occupied 
the  minds  of  men. 

i.  Pre formation. — When  the  mind  is  once  lost  in  the  mystery 
of  this  ever-recurring  miracle  it  is  not  surprising  to  find  that 
there  have  been  those  who  have  refused  to  believe  it  possible  and 
who  have  practically  denied  development  altogether.  The  old  doc- 
trine of  "evolution,"  as  it  was  called  by  the  scientists  of  the 
eighteenth  century,  or  of  preformation  as  we  know  it  to-day, 
held  that  all  the  organs  or  parts  of  the  adult  were  present  in  the 
germ  in  a  minute  and  transparent  condition  as  the  leaves  and 
stem  are  present  in  a  bud,  or  as  the  shoot  and  root  of  the  little 
plant  are  present  in  the  seed.*  In  the  case  of  animals  it  was 
generally  impossible  to  see  the  parts  of  the  future  animal  in  the 
germ,  but  this  was  supposed  to  be  due  to  the  smaller  size  of  the 
parts  and  to  their  greater  transparency,  and  with  poor  micro- 
scopes and  good  imaginations  some  observers  thought  they  could 
see  the  little  animal  in  the  egg  or  sperm,  and  even  the  little  man, 
or  "homunculus,"  was  described  and  figured  as  folded  up  in  one 
or  the  other  of  the  sex  cells. 

This  doctrine  of  preformation  was  not  only  an  attempt  to  solve 
the  mystery  of  development,  but  it  was  also  an  attempt  to  avoid 
the  theological  difficulties  supposed  to  be  involved  in  the  view 
that  individuals  are  produced  by  a  process  of  natural  develop- 
ment rather  than  by  supernatural  creation.  If  every  individual 
of  the  race  existed  within  the  germ  cells  of  the  first  parents,  then 
in  the  creation  of  the  first  parents  the  entire  race  with  its  millions 
of  individuals  was  created  at  once.  Thus  arose  the  theory  of 
"emboitement,"  or  infinite  encasement,  the  absurdities  of  which 

*  The  little  plant  in  the  seed  is  itself  the  product  of  the  development  of 
a  single  cell,  the  ovum,  in  which  no  trace  of  a  plant  is  present,  but  of  course 
this  fact  was  not  known  until  after  careful  microscopical  studies  had  been 
made  of  the  earliest  stages  of  development. 


58  Heredity  and  Environment 

contributed  to  the  downfall  of  the  entire  doctrine  of  preformation, 
which,  in  the  form  given  it  by  many  naturalists  of  the  eighteenth 
century,  is  now  only  a  curiosity  of  biological  literature. 

2.  Epigenesis. — As  opposed  to  .this  doctrine  of  preformation, 
which  was  founded  largely  on  speculation,  arose  the  theory  of 
epigenesis,  which  was  in  its  main  features  founded  upon  the  di- 
rect observation  of  development,  and  which  maintained  that  the 
germ  contains  none  of  the  adult  parts,  but  that  it  is  absolutely 
simple  and  undffferentiated,  and  that  from  these  simple  begin- 
nings the  individual  gradually  becomes  complex  by  a  process  of 
differentiation.    We  owe  the  "theory  of  epigenesis  at  least  so  far 
as  its  main  features  are  concerned,  to  William  Harvey,  the  dis- 
coverer of  the  circulation  of  the  blood,  and  to  Caspar  Friedrich 
Wolff,   whose   doctoral   thesis,   published   in    1759   and   entitled 
"Theoria  Generationis"  marked  the  beginning  of  a  great  epoch 
in  the  study  of  development.    Wolff  demonstrated  that  adult  parts 
are  not  present  in  the  germ,  either  in  animals  or  in  plants,  but 
that  these  parts  gradually  appear  in  the  process  of  development. 
He  held,  erroneously,  that  the  germ  is  absolutely  simple,  homo- 
geneous and  undifferentiated,  and  that  differentiation  and  or- 
ganization gradually  appear  in  this  undifferentiated  substance. 
How  to  get  differentiations  out  of  non-differentiated  material, 
heterogeneity  out  of  homogeneity,  was  the  great  problem  which 
confronted  Wolff  and  his  followers,  and  they  were  compelled  to 
assume  some  extrinsic  or  environmental  force,  some  vis  formativa 
or  spiritus  rector,  which  could  set  in  motion  and  direct  the  process 
of  development. 

The  doctrine  of  preformation,  by  locating  in  the  germ  all  the 
parts  which  would  ever  arise  from  it,  practically  denied  develop- 
ment altogether;  epigenesis  recognized  the  fact  of  development, 
but  attributed  it  to  mysterious  and  purely  hypothetical  external 
forces ;  the  one  placed  all  emphasis  upon  the  germ  and  its  struc- 
tures, the  other  upon  outside  forces  and  conditions. 

3.  Endogenesis  and  Epigenesis. — Modern  students  of  develop- 
ment recognize  that  neither  of  these  extreme  views  is  true — adult 


Facts  and  Factors  of  Development  59 

parts  are  not  present  in  the  germ,  nor  is  the  latter  homogeneous — 
but  there  are  in  germ  cells  many  different  structures  and  func- 
tions which  are,  however,  very  unlike  those  of  the  adult,  and  by 
the  transformation  and  differentiation  of  this  germinal  organi- 
zation the  complicated  organization  of  the  adult  arises.  Develop- 
ment is  not  the  unfolding  of  an  infolded  organism,  nor  the  mere 
sorting  of  materials  already  present  in  the  germ  cells,  though 
this  does  take  place,  but  rather  it  consists  in  the  formation  of  new 
materials  and  qualities,  of  new  structures  and  functions — by  the 
combination  and  interaction  of  the  germinal  elements  present  in 
the  oosperm.  In  similar  manner  the  combination  and  interaction 
of  chemical  elements  yield  new  substances  and  qualities  which  are 
not  to  be  observed  in  the  elements  themselves.  Such  new  sub- 
stances and  qualities,  whether  in  the  organic  or  in  the  inorganic 
world,  do  not  arise  by  the  gradual  unfolding  of  what  was  present 
from  the  beginning,  but  they  are  produced  by  a  process  of  "crea- 
tive synthesis." 

Modern  studies  of  germ  cells  have  shown  that  they  are  much 
more  complex  than  was  formerly  believed  to  be  the  case;  they 
may  even  contain  different  "organ-forming  substances"  which  in 
the  course  of  development  give  rise  to  particular  organs;  these 
substances  may  be  so  placed  in  the  egg  as  to  foreshadow  the 
polarity,  symmetry  and  pattern  of  the  embryo,  but  even  the  most 
highly  organized  egg  is  relatively  simple  as  compared  with  the 
animal  into  which  it  ultimately  develops.  Increasing  complexity, 
which  is  the  essence  of  development,  is  caused  by  the  combina- 
tion and  interaction  of  germinal  substances  under  the  influence 
of  the  environment.  The  organization  of  the  oosperm  may  be 
compared  to  the  arrangement  of  tubes  and  flasks  in  a  complicated 
chemical  operation ;  they  stand  in  a  definite  relation  to  one  another 
and  each  contains  specific  substances.  The  final  result  of  the 
operation  depends  not  merely  upon  the  substances  used,  nor 
merely  upon  the  way  in  which  the  apparatus  is  set  up,  but  upon 
both  of  these  things  as  well  as  upon  the  environmental  condi- 


60  Heredity  and  Environment 

tions  represented  by  temperature,  pressure,  moisture  or  other 
extrinsic  factors. 

4.  Heredity  and  Environment. — Unquestionably  the  factors 
or  causes  of  development  are  to  be  found  not  merely  in  the  germ 
but  also  in  the  environment,  not  only  in  intrinsic  but  also  in 
extrinsic  forces;  but  it  is  equally  certain  that  the  directing  and 
guiding  factors  of  development  are  in  the  main  intrinsic,  and 
are  present  in  the  organization  of  the  germ  cells,  while  the  en- 
vironmental factors  exercise  chiefly  a  stimulating,  inhibiting  or 
modifying  influence  on  development.  In  the  same  dish  and  un- 
der similar  environmental  conditions,  one  egg  will  develop  into  a 
worm,  another  into  a  sea  urchin,  another  into  a  fish,  and  it  is 
certain  that  the  different  fate  of  each  egg  is  determined  by  con- 
ditions intrinsic  in  the  egg  itself,  rather  than  by  environmental 
conditions.  We  should  look  upon  the  germ  as  a  living  thing,  and 
upon  development  as  one  of  its  functions.  Just  as  the  character 
of  any  function  is  determined  by  the  organism,  though  it  may  be 
modified  by  environment,  so  the  character  of  development  is 
determined  by  heredity,  i.e.,  by  the  organization  of  the  germ 
cells,  though  the  course  and  results  of  development  may  be  modi- 
fied by  environmental  conditions. 

SUMMARY 

In  conclusion,  we  have  briefly  reviewed  in  this  chapter  the  well 
known  fact  that  every  living  thing  in  the  world  has  come  into 
existence  by  a  process  of  development ;  that  the  entire  human  per- 
sonality, mind  as  well  as  body,  has  thus  arisen;  and  that  the 
factors  of  development  may  be  classified  as  intrinsic  in  the  organi- 
zation of  the  germ  cells,  and  extrinsic  as  represented  in  environ- 
mental forces  and  conditions.  The  intrinsic  factors  are  those 
which  are  commonly  called  heredity,  and  they  direct  and  guide 
development  in  the  main ;  the  extrinsic  or  environmental  factors 
furnish  the  conditions  in  which  development  takes  place  and 
they  modify,  more  or  less,  its  course. 


CHAPTER  II 
PHENOMENA  OF  INHERITANCE 


CHAPTER  II 


PHENOMENA   OF   INHERITANCE 

A.     OBSERVATIONS   ON    INHERITANCE 

The  observations  of  men  for  ages  past  have  established  the  fact 
that  in  general  "like  produces  like,"  and  that,  in  spite  of  many 
exceptions,  children  are  in  their  main  characteristics  like  their 
parents.  And  yet  offspring  are  never  exactly  like  their  parents, 
and  this  has  led  to  the  saying  that  "like  does  not  produce  like 
but  only  somewhat  like."  What  is  meant  is  that  there  are  gen- 
eral resemblances  but  particular  differences  between  parents  and 
offspring. 

INDIVIDUALS  AND  THEIR  CHARACTERS 

In  considering  organic  individuals  one  may  think  of  them  as 
wholes  or  as  composed  of  parts,  as  indivisible  unities  or  as  constit- 
uent characters;  either  aspect  is  a  true  one  and  yet  neither  is 
complete  in  itself.  Formerly  in  discussions  on  heredity  the 
individual  was  regarded  in  its  entirety  and  when  all  hereditary 
resemblances  and  differences  were  averaged  it  was  said  that  one 
child  resembled  the  father,  another  child  the  mother.  This 
method  of  lumping  together  and  averaging  resemblances  and  dif- 
ferences led  to  endless  confusion.  In  heredity  no  less  than 
in  anatomy  it  is  necessary  to  deal  with  the  constituents  of  organ- 
isms; in  short,  the  organism  must  be  analyzed  and  each  part 
studied  by  itself. 

Method  of  Gallon  and  Mendel. — Francis  Galton  was  one  of  the 

63 


64  Heredity  and  Environment 

first  to  bring  order  out  of  chaos  by  dealing  with  traits  or  char- 
acters singly  instead  of  treating  all  together.  He  made  careful 
studies  on  the  inheritance  of  weight  and  size  in  the  seeds  of 
sweet  peas,  and  on  the  inheritance  of  stature,  eye-color,  intel- 
lectual capacity,  artistic  ability  and  certain  diseases  in  man.  At 
the  same  time  that  Galton  was  thus  laying  the  foundations  for  a 
scientific  study  of  heredity  by  dealing  with  characters  separately, 
another  and  an  even  greater  student  of  heredity,  Gregor  Mendel, 
was  doing  the  same  thing  in  his  experiments  with  garden  peas, 
but  inasmuch  as  Mendel's  work  remained  practically  unknown 
for  many  years,  Galton  has  been  rightly  recognized  as  the  founder 
of  the  scientific  study  of  heredity. 

Of  course,  neither  Galton  nor  anyone  else  who  has  followed 
his  method  of  dealing  with  the  characters  of  organisms  singly,  ever 
supposed  that  such  characters  could  exist  independently  of  other 
characters  and  apart  from  the  entire  organism.  This  is  such  a 
self-evident  fact  that  it  may  seem  needless  to  mention  it,  and  yet 
there  have  been  critics  who  have  believed,  or  have  assumed  to 
believe,  that  modern  students  of  heredity  attempt  to  analyze  or- 
ganisms into  independently  existing  characters,  whereas  in  most 
cases  they  have  done  only  what  the  anatomist  does  in  treating 
-separately  the  various  organs  of  the  body. 

HEREDITARY  RESEMBLANCES  AND  DIFFERENCES 

The  various  characters  into  which  an  organism  may  be  analyzed 
show  a  greater  or  smaller  degree  of  resemblance  to  the  corre- 
sponding characters  of  its  parents.  Whenever  the  differential 
cause  of  a  character  is  a  germinal  one  the  character  is,  by  defini- 
tion, inherited ;  on  the  other  hand,  whenever  this  differential  cause 
is  environmental  the  character  is  not  inherited.  While  it  J*s  true 
that  inheritance  is  most  clearly  recognized  in  those  characters  in 
which  offspring  resemble  their  parents,  even  characters  in  which 
they  differ  from  their  parents  may  be  inherited,  as  is  plainly 
seen  when,  in  any  character,  a  child  resembles  a  grandparent  or  a 


Phenomena  of  Inheritance  65 

more  distant  ancestor  more  than  either  (parent.  Sometimes 
actually  new  characters  arise  in  descendants  which  were  not 
present  in  ascendants,  but  which  are  thereafter  inherited.  Ac- 
cordingly inherited  characters  may  be  classified  as  resemblances 
and  differences,  though  both  are  determined  by  germinal  organi- 
zation, or  heredity.  There  is  therefore  no  fundamental  difference 
between  inherited  similarities  and  dissimilarities.  Heredity  and 
variation  are  not  opposing  nor  contrasting  tendencies  which  make 
offspring  like  their  parents  in  one  case  and  unlike  them  in  an- 
other; really  inherited  characters  may  be  like  or  unlike  those  of 
the  parents. 

On  the  other  hand  many  resemblances  and  differences  between 
parents  and  offspring  are  due  not  to  heredity  at  all,  but  to  environ- 
mental conditions.  By  means  of  experiment  it  is  possible  to  dis- 
tinguish between  hereditary  and  environmental  resemblances  and 
differences,  but  among  men  where  experiments  are  generally  out 
of  the  question  it  is  often  difficult  or  impossible  to  make  this  dis- 
tinction. 

I.     HEREDITARY  RESEMBLANCES 

1.  Racial  Characters. — All  peculiarities  which  are  characteris- 
tic of  a  race,  species,  genus,  order,  class  and  phylum  are  of  course 
inherited,  otherwise  there  would  be  no  constant  characteristics  of 
these  groups  and  no  possibility  of  classifying  organisms.     The 
chief  characters  of  every  living  thing  are  unalterably  fixed  by 
heredity.    Men  do  not  gather  grapes  of  thorns  nor  figs  of  thistles. 
Every  living  thing  produces  offspring  after  its  own  kind.     Men, 
horses,  cattle;  birds,  reptiles,  fishes;  insects,  mollusks,  worms; 
polyps,  sponges,  micro-organisms, — all  of  the  million  known  spe- 
cies of  animals  and  plants  differ  from  one  another  because  of 
inherited  peculiarities,  because  they  have  come  from  different 
kinds  of  germ  cells  or  protoplasm. 

2.  Individual  Characters. — Many  characters  which  are  pecu- 
liar to  certain  individuals  are  known  to  be  inherited,  and  in  gen- 


66  Heredity  and  Environment 

eral  use  the  word  "inheritance"  refers  to  the  repetition  in  suc- 
cessive generations  of  such  individual  peculiarities.  Among  such 
individual  characters  are  the  following: 

(a)  Morphological  Features. — Hereditary  resemblances  are 
especially  recognizable  in  the  gross  and  minute  anatomy  of  every 
organism,  in  the  form,  structure,  location,  size,  color,  etc.,  of 
each  and  every  part.  The  number  of  such  individual  peculiarities 
which  are  inherited  is  innumerable  and  only  a  few  of  the  more 
striking  of  these  can  be  mentioned. 

It  is  a  matter  of  common  knowledge  that  unusually  great  or 
small  stature  runs  in  certain  families,  and  Galton  developed  a 
formula  for  determining  the  approximate  stature  of  children  from 
the  known  stature  of  the  parents  and  from  the  mean  stature  of 
the  race  (Fig.  25).  However,  his  statistical  and  mathematical 
formulae  give  only  general  or  average  results,  from  which  there 
are  many  individual  departures  and  exceptions. 

In  the  same  way  the  color  of  the  skin,  the  color  and  form  of 
hair  and  the  color  of  eyes  are  in  general  like  those  of  one  or 
more  of  the  parents  or  grandparents.  We  all  know  that  certain 
facial  features  such  as  the  shape  and  size  of  eyes,  nose,  mouth 
and  chin  are  generally  characteristic  of  certain  families. 

But  the  inheritance  of  anatomical  features  extends  to  much 
more  minute  characters  than  those  just  mentioned.  In  certain 
families  a  few  hairs  in  the  eyebrows  are  longer  than  the  others, 
or  there  may  be  patches  of  parti-colored  hair  over  the  scalp,  or 
dimples  in  the  cheek,  chin,  or  other  parts  of  the  skin  may 
occur,  and  these  trifling  peculiarities  are  inherited  with  all  the 
tenacity  shown  in  the  transmission  of  more  important  characters. 
Johannsen  has  found  races  of  beans  in  which  the  average  weight 
of  individual  seeds  differed  only  by  .02  to  .03  gram,  and  yet  these 
minute  differences  in  weight  were  characteristic  of  each  race  and 
were  of  course  inherited.  Jennings  has  found  races  of  Parame- 
cium  which  show  hereditary  differences  of  .005  mm.  in  average 
length  (Fig.  22).  Nettleship  says  that  the  lens  of  the  human  eye 


Phenomena  of  Inheritance  67 

weighs  only  175  milligrams,  or  about  one  three-millionth  part  of 
the  body  weight,  and  in  hereditary  cataract  only  about  one  twen- 
tieth part  of  the  lens  becomes  opaque,  and  yet  this  minute  frac- 
tion of  the  body  weight  shows  the  influence  of  heredity.  Even 
the  size,  shape  and  number  of  the  cells  in  certain  organs,  and  in 
given  embryonic  stages,  may  be  repeated  generation  after  gener- 
ation; and  if  our  analysis  were  sufficiently  complete  we  should 
doubtless  find  that  even  the  minute  parts  of  cells,  such  as  nuclei, 
chromosomes  and  centrosomes,  show  individual  peculiarities  which 
are  inherited. 

(b)  Physiological  peculiarities  are  inherited  as  well  as  morpho- 


105 


43 


FIG.  22.  DIAGRAM  OF  EIGHT  DIFFERENT  RACES  OF  Paramedum,  each  hori- 
zontal row  (A-H)  representing  a  single  race.  The  individual  showing  the 
mean  size  in  each  race  is  indicated  by  -\- ;  the  mean  of  all  the  races  is  shown 
by  the  line  X-X.  The  numbers  are  the  lengths  in  micra  (thousandths  of 
a  millimeter),  X  43-  (After  Jennings.) 


68  Heredity  and  Environment 

logical  ones;  indeed  function  and  structure  are  only  two  aspects 
of  one  and  the  same  thing,  namely  organization.  For  all  morpho- 
logical characters  there  are  functional  correlatives,  for  functional 
characters  morphological  expressions,  and  if  the  one  is  inherited 
so  is  the  other.  But  there  are  certain  characters  in  which  the 
physiological  aspect  is  more  striking  than  the  morphological  one. 

Longevity. — For  example,  longevity  is  a  physiological  character 
which  is  undoubtedly  dependent  upon  many  causes,  but  in  the 
case  of  species  which  differ  greatly  in  length  of  life  there  can  be 
little  doubt  that  we  are  dealing  with  an  inherited  character.  The 
great  differences  in  the  length  of  life  of  an  elephant  and  a  mouse, 
of  a  parrot  and  a  pigeon,  of  a  cicada  and  a  squash  bug,  are  as  surely 
the  result  of  inherited  causes  as  are  the  structural  differences  be- 
tween these  animals.  Within  the  same  species  different  races 
or  lines  show  characteristic  differences  in  length  of  life;  in  the 
case  of  man  the  average  length  of  life  is  much  greater  in  some 
families  than  in  others,  and  life-insurance  companies  take  account 
of  this  fact.  Even  within  the  same  organism  certain  organs  or 
cells  are  short-lived,  whereas  others  are  long-lived;  some  cells 
and  organs  live  only  through  the  early  embryonic  period,  while 
others  live  as  long  as  the  general  organism. 

Other  Functional  Characters. — Obesity  is  another  physiological 
characteristic  which  may  be  inherited;  the  members  of  certain 
families  grow  fat  in  spite  of  themselves,  while  members  of  other 
families  remain  thin  however  well  fed  they  may  be.  Here  also 
many  factors  enter  into  the  result,  but  it  seems  probable  that  the 
differentiating  factor  is  an  hereditary  one.  Baldness  affects  the 
male  members  of  certain  families  when  they  have  reached  a  given 
age,  while  in  others  neither  care,  dissipation  nor  age  can  rob  a 
man'of  his  bushy  top.  Haemophilia,  or  excessive  bleeding  after  an 
injury,  which  is  due  to  a  deficiency  in  the  clotting  power  of  the 
blood,  is  strongly  inherited  in  the  male  line  in  certain  families. 
Fecundity  and  a  tendency  to  bear  twins  or  triplets,  left-handed- 
ness,  a  peculiar  lack  of  resistance  to  certain  diseases,  and  many 
other  physiological  peculiarities  are  probably  inherited. 


Phenomena  of  Inheritance  69 

(c)  Pathological  Peculiarities  are  really  only  unusual  or  abnor- 
mal anatomical  or  physiological  characters,  but  they  are  of  such 
interest  and  importance  as  to  deserve  special  mention.  Many  such 
abnormalities  are  undoubtedly  inherited,  among  which  are  the 
following :  polydactylism,  in  which  more  than  the  normal  number 
of  digits  are  present  (Fig.  38)  ;  syndactylism,  or  a  condition 
of  webbed  fingers  and  toes ;  brachydactylism,  in  which  fingers  are 
short  and  stumpy  and  usually  contain  less  than  the  normal  num- 
ber of  joints  (Fig.  39)  ;  achondroplasy,  or  short  and  crooked 
limbs,  such  as  occur  in  certain  breeds  of  dogs  and  sheep  and  in 
certain  human  dwarfs ;  myopia,  in  which  the  eyeball  is  elongated  ; 
glaucoma,  or  pressure  within  the  eyeball ;  coloboma,  or  open  su- 
ture of  the  iris ;  otosclerosis,  or  rigidity  of  tympanum  and  ossicles, 
causing  "hardness  of  hearing" ;  some  forms  of  deaf-mutism,  due 
to  certain  defects  of  the  inner  ear ;  and  many  other  characters  too 
numerous  to  mention  here.  On  the  other  hand  many  abnormal 
or  monstrous  conditions  are  due  to  abnormal  environment  and  are 
not  inherited. 

Are  Diseases  Inherited. — The  question  of  the  inheritance  of 
diseases  may  be  briefly  considered  here.  If  a  disease  is  due  to 
some  defect  in  the  hereditary  constitution,  it  is  inherited ;  other- 
wise, according  to  our  definition  of  heredity,  it  is  not.  Of  course 
no  disease  develops  without  extrinsic  causes  but  when  one  indi- 
vidual takes  a  disease  while  another  under  the  same  conditions 
does  not,  the  differential  cause  may  be  an  inherited  one,  or  it  may 
be  due  to  differences  in  the  previous  conditions  of  life.  There 
is  no  doubt  that  certain  diseases  run  in  families  and  have  the  ap- 
pearance of  being  inherited,  but  in  this  case  as  in  many  others  it 
is  extremely  difficult  in  the  absence  of  experiments  to  distinguish 
between  effects  due  to  intrinsic  causes  and  those  due  to  extrinsic 
ones.  Where  the  specific  cause  of  disease  is  some  micro-organism 
the  individual  must  have  been  infected  at  some  time  or  other,  al- 
most invariably  after  birth.  In  few  instances  is  the  oosperm  itself 
infected,  and  even  when  it  is,  this  is  not,  strictly  speaking,  a  case 


70  Heredity  and  Environment 

of  inheritance,  but  rather  one  of  early  infection.  Leo  Loeb  has 
shown  that  cancer  is  inherited  in  mice  and  Little  finds  that  there 
is  inheritance  of  a  predisposition  to  cancer  in  man.  Pearson  has 
found  that  there  is  a  marked  correlation  (represented  by  the  num- 
ber .55  when  complete  correlation  is  I.)  between  tuberculous 
parents  and  tuberculous  children,  but  there  is  very  little  evidence 
that  the  child  is  ever  infected  before  birth.  What  is  inherited  in 
this  case  is  probably  slight  resistance  to  the  tubercle  bacillus. 
There  is  evidence  that  almost  all  adult  persons  have  been  infected 
at  one  time  or  another  by  this  bacillus,  but  it  has  not  developed 
far  in  all  of  them  because  some  have  superior  powers  of  resistance. 
Such  greater  or  smaller  resistance,  stronger  or  weaker  build,  is 
inherited,  and  while  diminished  resistance  is  not  the  direct  cause 
of  tuberculosis  it  is  a  predisposing  cause.  The  same  is  probably 
true  of  many  other  diseases,  the  immediate  causes  of  which  are 
extrinsic,  while  only  the  more  remote,  or  predisposing  causes, 
are  hereditary. 

(d)  Psychological  Characters  appear  to  be  inherited  in  the 
same  way  that  anatomical  and  physiological  traits  are ;  indeed  all 
that  has  been  said  regarding  the  correlation  of  morphological  and 
physiological  characters  applies  also  to  psychological  ones.  No 
one  doubts  that  particular  instincts,  aptitudes  and  capacities  are 
inherited  among  both  animals  and  men,  nor  that  different  races 
and  species  differ  hereditarily  in  psychological  characteristics. 

Certain  breeds  of  dogs  such  as  the  mastiff,  the  bull  dog,  the  ter- 
rier, the  collie  and  many  others  are  characterized  by  peculiarities 
of  temperament,  affection,  intelligence  and  disposition.  No  one 
who  has  much  studied  the  subject  can  doubt  that  different  human 
races  and  families  show  characteristic  differences  in  these  same 
respects.  It  is  quite  futile  to  argue  that  exceptional  individuals 
may  be  found  in  one  race  with  the  mental  characteristics  of  an- 
other race ;  the  same  could  be  said  of  different  breeds  of  dogs,  or 
of  the  sizes  of  different  races  of  beans  or  of  Paramecia  (Fig.  22). 
The  fact  is  that  racial  characteristics  are  not  determined  by  excep- 


Phenomena  of  Inheritance  71 

tional  and  extreme  individuals  but  by  the  average  or  mean  quali- 
ties of  the  race;  and  measured  in  this  way  there  is  no  doubt  that 
certain  types  of  mind  and  disposition  are  characteristic  of  certain 
families. 

There  is  no  longer  any  question  that  some  kinds  of  f  eeble-mind- 
edness,  epilepsy  and  insanity  are  inherited,  and  that  there  is  often 
an  hereditary  basis  for  nervous  and  phlegmatic  temperaments,  for 
emotional,  judicial  and  calculating  dispositions.  Nor  can  it  be 
denied  that  strength  or  weakness  of  will,  a  tendency  to  moral 
obliquity  or  rectitude,  capacity  or  incapacity  for  the  highest  in- 
tellectual pursuits,  occur  frequently  in  certain  families  and  ap- 
pear to  be  inherited.  In  spite  of  certain  noteworthy  exceptions, 
which  may  perhaps  be  due  to  remarkable  variations,  statistics  col- 
lected by  Galton  show  that  genius  runs  in  certain  families ;  while 
the  work  of  some  recent  investigators,  particularly  Goddard, 
Davenport  and  Weeks,  proves  that  f  eeble-mindedness  and  epilepsy 
are  also  inherited;  and  the  careful  work  of  Mdtt  and  of  Rosanoff 
indicates  that  certain  types  of  insanity  are  hereditary.  On  the 
other  hand,  Cotton  maintains  that  mental  disorders  are  not  di- 
rectly inherited,  but  that  "there  is  probably  a  constitutional  lack 
of  resistance  to  various  toxins  and  poisons,  and  not  an  inherited 
mental  instability,  which  causes  the  mind  to  break  down  under 
mental  stress  and  strain."  It  frequently  happens  that  families 
in  which  hereditary  insanity  occurs  also  have  other  members 
afflicted  with  epilepsy,  hysteria,  alcoholism,  etc.,  which  seem  to 
indicate  that  the  thing  inherited  is  an  unstable  condition  of  the 
nervous  system  which  may  take  various  forms  under  slightly 
different  conditions.  Indeed  there  is  a  good  deal  of  evidence  that 
extraordinary  ability,  or  genius  is  frequently  associated  with  an 
unstable  nervous  organization  which  sometimes  takes  the 
form  of  insanity  or  epilepsy  or  alcoholism.  There  is  perhaps 
more  truth  than  poetry  in  Dryden's  lines: 

"Great  wits  are  sure  to  madness  near  allied, 
And  thin  partitions  do  their  bounds  divide." 


72  Heredity  and  Environment 

Woods  has  collected  data  concerning  "Heredity  in  Royalty" 
which  seem  to  show  that  very  high  or  low  grades  of  intellect  and 
morality  may  be  traced  through  the  royal  families  of  Europe  for 
several  generations.  Extensive  study  of  certain  families  in  which 
an  extraordinary  number  of  feeble-minded,  degenerate,  and  crim- 
inal individuals  have  appeared,  seems  to  demonstrate  that  moral 
and  social  qualities  are  also  inherited.  One  recalls  in  this  con- 
nection the  famous,  or  rather  infamous,  "Jukes",  "Kalikaks", 
"Nams",  and  "Ishmaels", — these  names  being  pseudonymns  for 
notoriously  bad  families  whose  traits  have  been  followed  through 
several  generations. 

The  general  tendency  of  recent  work  on  heredity  is  unmistak- 
able, whether  it  concerns  man  or  lower  animals.  The  entire  or- 
ganism, consisting  of  structures  and  functions,  body  and  mind, 
develops  out  of  the  germ,  and  the  organization  of  the  germ  deter- 
mines all  the  possibilities  of  development  of  the  mind  no  less  than 
of  the  body,  though  the  actual  realization  of  any  possibility  is 
dependent  also  upon  environmental  stimuli. 

II.    HEREDITARY  DIFFERENCES 

There  are  many  exceptions  to  the  general  rule  that  children 
resemble  their  parents ;  indeed  no  child  is  ever  exactly  like  a  par- 
ent and  the  points  in  which  they  differ  are  known  generally  as 
variations.  These  variations  are  of  two  kinds,  those  which  are 
caused  by  a  different  germinal  constitution  and  are  therefore  in- 
herited and  those  due  to  environmental  differences  which  are  not 
inherited.  Sometimes  inherited  variations  are  due  to  new  com- 
binations of  ancestral  characters,  sometimes  they  are  actually  new 
characters  not  present  so  far  as  known  in  any  of  the  ancestors, 
though  even  such  new  characters  must  arise  from  new  c'ombina- 
tions  of  the  elements  of  old  characters,  as  we  shall  see  later. 

i.  New  Combinations  of  Characters. — In  all  cases  of  sexually 
produced  organisms  new  combinations  of  ancestral  characters  are 
evident.  Usually  a  child  inherits  some  traits  from  one  parent 


Phenomena  of  Inheritance  73 

and  other  traits  from  the  other  parent,  so  that  it  is  a  kind  of 
mosaic  of  ancestral  traits.  Such  inheritance,  bit  by  bit,  of  this 
character  from  one  progenitor  and  that  from  another  was  de- 
scribed by  Galton  as  "particulate"  (Fig.  23)  and  is  known  today 
as  "Mendelian."  As  we  shall  see  later  (p.  88  et seq.)  this  is  prob- 
ably the  only  kind  of  inheritance.  On  the  other  hand  Galton 
supposed  that  in  some  instances  a  child  might  inherit  all  or  nearly 
all  of  his  traits  from  one  parent,  a  thing  which  is  most  improb- 
able; such  inheritance  he  called  "alternative"*  (Fig.  23). 

In  other  cases  the  traits  of  the  parents  appear  to  blend  in  the 
offspring,  as  for  example,  in  the  skin  color  of  mulattoes;  such 

BLENDING  ALTERNATIVE  PARTICULATE 


FIG.  23.  DIAGRAM  TO  ILLUSTRATE  THREE  KINDS  OF  INHERITANCE  described 
by  Galton.  Only  the  last  of  these  (particulate)  really  occurs.  (After 
Walter.) 

cases  were  called  by  Galton  "blending"  inheritance  (Fig.  23). 
Such  cases  of  blending  inheritance  are  now  known  to  be  the 
result  of  particulate  inheritance  of  many  factors  (p.  108).  Some- 
times characters  appear  in  offspring  which  were  "latent"  in  the 
parents  but  were  "patent"  in  one  or  more  of  the  grandparents ; 

*  It  is  necessary  to  distinguish  between  alternative  inheritance  of  a  single 
character  (Mendel)  and  this  supposed  alternative  inheritance  of  all  char 
acters  (Galton), 


74  Heredity  and  Environment 

such  skipping  of  a  generation,  during  which  a  character  re- 
mains "latent,"  has  long  been  known  as  "atavism."  At  other 
times  characters  which  were  present  in  distant  ancestors,  but 
which  have  since  dropped  out  of  sight  or  have  remained  "latent," 
reappear  in  descendants ;  such  cases  are  known  as  "reversions." 

In  still  other  cases  certain  characters  appear  only  in  the  male 
sex,  others  only  in  the  female,  this  being  called  "sex-limited"  in- 
heritance ;  while  in  some  instances  characters  are  transmitted  from 
fathers  through  daughters  to  grandsons  or  from  mothers  to  sons, 
all  such  cases  being  known  as  "sex-linked"  inheritance.* 

2.  New  Characters  or  Mutations. — But  in  addition  to  these 
permutations  in  the  distribution  and  combination  of  ancestral 
characters  new  and  unexpected  characters  sometimes  develop 
in  the  offspring,  which  were  not  present,  so  far  as  known,  in  any 
of  the  ascendants,  but  which,  after  they  have  once  appeared,  are 
passed  on  by  heredity  to  descendants.  Such  inherited  variations 
are  usually  of  two  kinds,  continuous  or  slight,  and  discontinuous 
or  "sudden"  variations.  The  latter  are  especially  noticeable  when 
variations  occur  in  the  normal  number  of  parts,  as  in  four-leaved 
clover,  or  six-fingered  men,  and  such  numerical  variations  have 
been  called  by  Bateson  "meristic."  However,  sudden  variations 
may  include  any  marked  departure  from  the  normal  type,  in  color, 
shape,  size,  chemical  composition,  etc.  Such  sudden  variations 
have  long  been  known  to  breeders  as  "sports,"  and  both  Darwin 
and  Galton  pointed  out  the  fact  that  such  sports  have  sometimes 
given  rise  to  new  races  or  breeds,  though  Darwin  was  not  in- 
clined to  assign  much  importance  to  them  in  the  general  process 
of  evolution.  Galton,  on  the  other  hand,  maintained  that  varia- 
tions, or  what  would  now  be  called  "continuous  variations,"  can- 
not be  of  much  significance  in  the  process  of  evolution,  but  that 
the  case  is  quite  different  with  "sports"  ("Hereditary  Genius," 
prefatory  chapter). 

More  recently  the  entire  biological  world  has  been  greatly  influ- 

*See  p.  187. 


Phenomena  of  Inheritance  75 

enced  by  the  "Mutation  Theory"  of  deVries,  which  has  placed  a 
new  emphasis  upon  the  importance  of  sudden  variations  in  the 
process  of  evolution.  At  first  deVries  was  inclined  to  emphasize 
the  degree  of  difference,  that  is  the  discontinuity,  in  these  varia- 
tions, but  in  later  works  this  distinction  is  given  a  minor  place  as 
compared  with  the  question  whether  variations  are  inherited  or 
not.  Inherited  variations,  whether  large  or  small,  are  called  by 
deVries  "mutations,"  whereas  non-inherited  variations  are  known 
as  "fluctuations."  The  former  are  caused  by  changes  in  germinal 
constitution,  the  latter  by  alterations  in  environmental  conditions ; 
the  former  represent  changes  in  heredity,  the  latter  changes  in 
development. 

3.  Mutations  and  Flucttuftionis. — This  clear  cut  distinction  be- 
tween mutations  and  fluctuations  marks  one  of  the  most  impor- 
tant advances  ever  made  in  the  study  of  development  and  evolu- 
tion.   Thousands  of  fluctuations  occur  which  are  purely  somatic 
in  character  and  which  do  not  affect  the  germ  cells,  for  everv 
single  mutation  or  change  in  the  hereditary  constitution ;  and  yet 
only  the  latter  are  of  significance  in  heredity  and  evolution.    This 
distinction  between  variations  due  to  environment  ( fluctuations  )^ 
and  those  due  to  hereditary  causes  (mutations)  was  recognized 
by  Weismann  and  many  of  his  followers,  but  the  actual  demon- 
stration on  a  large  scale  of  the  importance  of  this  distinction  is 
due  mainly  to  deVries. 

All  hereditary  variations,  whether  due  to  new  combinations  of 
old  characters  or  to  the  appearance  of  actually  new  characters, 
whether  small  and  continuous  or  large  and  discontinuous,  have 
their  causes  in  the  organization  of  the  germ  cells,  just  as  do  in- 
herited resemblances.  Heredity  is  not  to  be  contrasted  with  var- 
iation, nor  are  hereditary  likeness  and  unlikeness  due  to  con- 
flicting principles;  both  are  the  results  of  germinal  organization 
and  both  are  phenomena  of  heredity. 

4.  Every  Individual  Unique. — As  a  result  of  the  permutations 
of  ancestral  characters,  the  appearance  of  mutations,  and  the 


76  Heredity  and  Environment 

fluctuations  of  organisms  due  to  environmental  changes,  it  hap- 
pens that  in  all  cases  offspring  differ  more  or  less  from  their  par- 
ents and  from  one  another.  No  two  children  of  the  same  family 
are  ever  exactly  alike  (except  in  the  case  of  identical  twins  which 
have  come  from  the  same  oosperm.*  Every  living  being  appears 
on  careful  examination  to  be  the  first  and  last  of  its  identical 
kind.  This  is  one  of  the  most  remarkable  peculiarities  of  living 
things.  The  elements  of  chemistry  are  constant,  and  even  the 
compounds  fall  into  definite  categories  which  have  constant  char- 
acteristics. But  the  individuals  of  biology  are  apparently  never 
twice  the  same.  This  may  be  due  to  the  immense  complexity  of 
living  units  as  contrasted  with  chemical  ones, — indeed  lack  of 
constancy  is  evidence  in  itself  of  lack  of  analysis  into  real  ele- 
ments or  of  lack  of  uniform  conditions, — but  whatever  its  cause 
the  extraordinary  fact  remains  that  every  living  being  appears  to 
be  unique.  •  "Reproduction  is  the  generation  of  unique  beings 
that  are,  on  the  average,  more  like  their  kind  than  like  anything 
else"  (Brooks). 

There  seems  to  be  no  reason  to  doubt  that  all  the  extraordinary 
.differences  which  organisms  show,  as  well  as  all  of  their  resem- 
blances, are  due  to  differences  or  resemblances  in  the  hereditary 
and  environmental  factors  which  have  been  operative  in  their 
development.    But  in  view  of  this  universal  variability  of  organ- 
isms it  is  not  surprising  that  inheritance  has  seemed  capricious  and 
uncertain, — "a  sort  of  maze  in  which  science  loses  itself." 
B.    STATISTICAL  STUDY  OF  INHERITANCE 

Francis  Galton  was  one  of  the  first  who  attempted  to  reduce  the 
mass  of  conflicting  observations  on  heredity  and  variation  to 
some  system  and  to  establish  certain  principles  as  a  result^)  f  sta- 
tistical study.  He  was  the  real  founder  of  the  scientific  study  of 
inheritance ;  he  studied  characters  singly  and  he  introduced  quan- 
titative measures.  Gallon's  researches,  which  were  published 
in  several  volumes,  consisted  chiefly  in  a  study  of  certain  families 

*  See  p.  229. 


Phenomena  of  Inheritance  77 

with  regard  to  several  selected  traits,  viz.,  genius  or  marked  in- 
tellectual capacity,  artistic  faculty,  stature,  eye  color  and  disease. 
As  a  result  of  his  very  extensive  studies  two  main  principles  ap- 
peared to  be  established : 

i.  The  Law  of  Ancestral  Inheritance  which  he  stated  as  fol- 
lows: 

The  two  parents  contribute  between  them  on  the  average  one- 
half  of  each  inherited  faculty,  each  of  them  contributing  one- 
quarter  of  it.  The  four  grandparents  contribute  between  them 
one-quarter,  or  each  of  them  one-sixteenth;  and  so  oh,  the  sum 
of  the  series  1/2  +  1/4  +  1/8  +  1/16  .  .  .  being  equal  to  I,  as 
it  should  be.  It  is  a  property  of  this  infinite  series  that  each  term 
is  equal  to  the  sum  of  all  those  that  follow:  thus  1/2  =  1/4  + 
!/8  +  1/16  +  ...,1/4=1/8+  1/16  +  .  .  v  and  so  on.  The 
prepotencies  of  particular  ancestors  in  any  given  pedigree  are 
eliminated  by  a  law  which  deals  only  with  average  contributions, 
and  the  various  prepotencies  of  sex  wiith  respect  to  different 
qualities  are  also  presumably  eliminated. 

The  average  contribution  of  each  ancestor  was  thus  stated 
definitely,  the  contribution  diminishing  with  the  remoteness  of 
the  ancestor.  This  Law  of  Ancestral  Inheritance  is  represented 
graphically  in  the  accompanying  diagram  (Fig.  24).  Pearson  has 
proposed  a  Law  of  Reversion  according  to  which  the  average  re- 
version of  offspring  to  each  ascending  generation  of  ancestors  is 
represented  by  the  series  .3,  .15,  .075,  .0375,  etc. 

Ancestors  and  Contributors. — Theoretically  the  number  of  an- 
cestors doubles  in  each  ascending  generation ;  there  are  two  par- 
ents, four  grandparents,  eight  great-grandparents,  etc.  If  this 
continued  to  be  true  indefinitely  the  number  of  ancestors  in  any 
ascending  generation  would  be  (2)°,  in  which  n  represents  the 
number  of  generations,  There  have  been  about  57  generations 
since  the  beginning  of  the  Christian  Era,  and  if  this  rule  held  true 
indefinitely  each  of  us  would  have  had  at  the  time  of  the  birth 
of  Christ  a  number  of  ancestors  represented  by  (2)57  or  about 
120  quadrillions, — a  number  far  greater  than  the  entire  human 
population  of  the  globe  since  that  time.  As  a  matter  of  fact, 


Heredity  and  Environment 


owing  to  the  intermarriage  of  cousins  of  various  degrees  the 
actual  number  of  ancestors  is  much  smaller  than  the  theoretical 
number.  For  example,  Plate  says  that  the  late  Emperor  of  Ger- 
many, Wilhelm  II,  had  only  162  ancestors  in  the  loth  ascend- 
ing generation,  instead  of  512,  the  theoretical  number.  Neverthe- 
less this  calculation  will  serve  to  show  how  widespread  our  an- 
cestral lines  are,  and  how  nearly  related  are  all  people  of  the 
same  race, 

Davenport  concludes  that  no  people  of  English  descent  are 
more  distantly  related  than  3Oth  cousins,  while  most  people  are 
much  more  closely  related  than  that.  If  we  allow  three  genera- 
tions to  a  century,  and  calculate  that  the  degree  of  cousinship  is 
determined  by  the  number  of  generations  less  two,  since/  first 
cousins  appear  only  in  the  third  generation,  the  first  being  that 
of  the  parents  and  the  second  that  of  the  sons  and  daughters,  we 
find  that  3Oth  cousins  at  the  present  time  would  have  had  a  com- 
mon ancestor  about  one  thousand  years  ago  or  approximately  at 
the  time  of  William  the  Conqueror.  As  a  matter  of  fact  most  per- 


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FIG.  24.  DIAGRAM  OF  GALTON'S  "LAW  OF  ANCESTRAL  INHERITANCE."  The 
whole  heritage  is  represented  by  the  entire  rectangle;  that  derived  from 
each  progenitor  by  the  smaller  squares;  the  number  of  the  latter  doubles  in 
each  ascending  generation  while  its  area  is  halved.  (After  Thompson.) 


Phenomena  of  Inheritance 


79 


sons  of  the  same  race  are  much  more  closely  related  than  this, 
and  certainly  we  need  not  go  back  to  Adam  nor  even  to  Shem, 
Ham,  or  Japheth  to  find  our  common  ancestor. 

On  the  other  hand  we  now  know  that  we  do  not  inherit  equally 
from  all  our  ancestors;  on  the  average  we  inherit  about  as  many 
traits  from  our  fathers  as  from  our  mothers,  but  inheritance  from 


63 


F.c.  25.  SCHEME  TO  ILLUSTRATE  GALTON'S  "LAW  OF  FILIAL  REGRESSION" 
as  shown  in  the  stature  of  parents  and  children.  The  mean  height  of 
all  parents  is  shown  by  the  dotted  line  between  68  and  69  inches.  The 
circles  through  which  the  diagonal  line  runs  represent  the  heights  of 
graded  grpups  of  parents  and  the  arrow  heads  indicate  the  average  heights 
of  their  children.  The  offspring  of  undersized  parents  are  taller  and  of 
oversized  parents  are  shorter  than  their  respective  parents.  (After 
Walter.) 


8o  Heredity  and  Environment 

the  four  grand-parents  is  usually  unequal  and  the  farther  back 
we  go  the  more  ancestors  we  find  who  have  contributed  nothing 
to  our  inheritance.  Of  all  the  thousands  or  even  millions  of  an- 
cestors that  each  of  us  has  had,  only  a  relatively  small  number 
have  contributed  anything  to  our  inheritance;  although  we  are 
descended  from  all  the  others  we  are  not  related  to  them  bio- 
logically and  have  received  none  of  their  traits.  Those  who  have 
contributed  to  our .  inheritance  may  be  called  "contributing  an- 
cestors" or  merely  "contributors"*  to  distinguish  them  from  non- 
contributing  ones,  and  the  fact  that  ancestors  do  not  contribute 
equally  to  heredity  disproves  Galton's  "law  of  ancestral  inherit- 
ance." 

2.  The  Law  of  Filial  Regression  is  the  second  principle  which 
Galton  deduced  from  his  statistical  studies,  or  it  may  be  called 
the  tendency  to  mediocrity.  He  found  that,  on  the  average,  ex- 
treme peculiarities  of  parents  were  less  extreme  in  children. 
Thus,  "the  stature  of  adult  offspring  must  on  the  whole  be  more 
mediocre  than  the  stature  of  their  parents,  that  is  to  say  more 
near  to  the  mean  or  mid  of  the  general  population" ;  and  again, 
"the  more  bountifully  a  parent  is  gifted  by  nature,  the  more  rare 
will  be  his  good  fortune  if  he  begets  a  son  who  is  as  richly  en- 
dowed as  himself."  This  so-called  law  of  filial  regression  is 
represented  graphically  in  Fig.  25  in  which  the  actual  stature  of 
individual  parents  is  shown  by  the  oblique  line,  the  stature  of 
children  by  the  dotted  curve,  and  the  mean  stature  of  the  race  in 
the  horizontal  dotted  line. 

Statistical  vs.  Physiological  Methods. — One  of  the  chief  aims 
and  results  of  statistical  studies  is  to  eliminate  individual  peculiari- 
ties and  to  obtain  general  and  average  results.  Such  work  may 
be  of  great  importance  in  the  study  of  heredity,  especially  where 
questions  of  the  occurrence  or  distribution  of  particular  phenomena 
are  concerned;  but  the  causes  of  heredity  are  individual  and 

*  I  have  adopted  this  term  proposed  by  Dr.  H.  H.  Laughlin  in  prefer- 
ence to  "transmitters"  which  I  had  previously  used. 


Phenomena  of  Inheritance  81 

physiological,  and  averages  are  of  less  value  in  finding  the  causes 
of  such  phenomena  than  is  the  intensive  study  of  individual 
families. 

By  observation  alone  it  is  usually  impossible  to  distinguish  be- 
tween inherited  and  environmental  resemblances  and  differences, 
and  yet  this  distinction  is  essential  to  any  study  of  inheritance.  If 
all  sorts  of  likenesses  or  unlikenesses  are  lumped  together, 
whether  inherited  or  not,  our  study  of  inheritance  can  only  end 
in  confusion.  The  value  of  statistics  depends  upon  a  proper 
classification  of  the  things  measured  and  enumerated,  and  if 
things  which  are  not  commensurable  are  grouped  together  the 
results  may  be  quite  misleading  and  worthless. 

Statistical  Studies  Insufficient. — Unfortunately  Galton  and 
Pearson,  as  well  as  some  of  their  followers,  have  not  always 
carefully  distinguished  between  hereditary  and  environmental 
characters.  Furthermore  much  of  their  material  was  drawn 
from  a  general  population  in  which  were  many  different  fam- 
ilies and  lines  not  closely  related  genetically.  Consequently  their 
statistical  studies  are  of  little  value  in  discovering  the  physio- 
logical principles  or  laws  of  heredity.  Jennings  (1910)  well  says, 
"Galton's  laws  of  regression  and  of  ancestral  inheritance  are  the 
product  mainly  of  a  lack  of  distinction  between  two  absolutely 
diverse  things,  between  non-inheritable  fluctuations  on  the  one 
hand,  and  permanent  genotypic  differentiations  on  the  other." 
In  the  case  of  man  we  have  few  certain  tests  to  determine  whether 
the  differential  cause  of  any  character  is  "hereditary  or  environ- 
mental, but  in  the  case  of  animals  and  plants,  where  experiments 
may  be  performed  on  a  large  scale,  it  is  possible  to  make  such 
tests  by  (i)  experiments  in  which  the  environment  is  kept  as 
uniform  as  possible  while  the  hereditary  factors  differ,  and  (2) 
experiments  in  which,  in  a  series  of  cases,  the  hereditary  factors 
are  fairly  constant  while  the  environment  differs.  In  this  way 
the  differential  cause  or  causes  of  any  character  may  be  located 
in  heredity,  in  environment  or  in  both. 


82  Heredity  and  Environment 

The  observational  and  statistical  study  of  inheritance  helped 
to  outline  the  problem  but  did  little  to  solve  it.  Certain  phenom- 
ena of  hereditary  resemblances  between  ascendants  and  descen- 
dants were  made  intelligible,  but  there  were  many  peculiar  and 
apparently  irregular  or  lawless  phenomena  which  could  not  be 
predicted  before  they  occurred  nor  explained  afterward.  For 
example  when  Darwin  crossed  different  breeds  of  domestic 
pigeons,  no  one  of  which  had  a  trace  of  blue  in  its  plumage,  he 
sometimes  obtained  offspring  with  more  or  less  of  the  blue  color 
and  markings  of  the  wild  rock  pigeon  from  which  domestic 
pigeons  are  presumably  descended.  He  described  many  cases 
of  dogs,  cattle  and  swine,  as  well  as  many  cultivated  plants,  in 
which  offspring  resembled  distant  ancestors  and  differed  from 
nearer  ones ;  such  cases  had  long  been  known  and  were  spoken 
of  as  "reversions."  He  observed  many  cases  in  which  certain 
characters  of  one  parent  prevailed  over  corresponding  characters 
of  the  other  parent  in  the  offspring,  this  being  known  as  "pre- 
potency"; but  there  was  no  satisfactory  explanation  of  these 
curious  phenomena.  They  did  not  come  under  either  of  Galton's 
"laws,"  and  their  occurrence  was  apparently  so  irregular  that 
every  such  case  seemed  to  be  a  law  unto  itself. 

C.     EXPERIMENTAL  STUDY  OF  INHERITANCE 

I.    MENDELISM 

The  year  1900  marks  the  beginning  of  a  new  era  in  the  study 
of  inheritance.  In  the  spring  of  that  year  three  botanists, 
deVries,  Correns,  and  Tschermak,  discovered  independently  an 
important  principle  of  heredity  and  at  the  same  time  brought  to 
light  a  long  neglected  and  forgotten  work  on  "Experiments  in 
Plant  Hybridization"  by  Gregor  Mendel,  in  which  this  same 
principle  was  set  forth  in  detail.  This  principle  is  now  generally 
known  as  "Mendel's  Law."  Mendel,  who  was  a  monk,  and  later 
abbot,  0*  the  Konigskloster,  an  Augustinian  monastery  in  Briinn, 
Moravia,  published  the  results  of  his  experiments  on  hybridization 


Phenomena  of  Inheritance  83 

in  the  Proceedings  of  the  Natural  History  Society  of  Briinn  in 
1866.  The  paper  attracted  but  little  attention  at  the  time  al- 
though it  contained  some  of  the  most  important  discoveries  re- 
garding inheritance  which  had  ever  been  made,  and  it  remained 
buried  and  practically  unknown  for  thirty-five  years.  Plant  hy- 
bridization jiad  been  studied  extensively  before  Mendel  began  his 
work,  but  he  carried  on  his  observations  of  the  hybrids  and  of 
their  .progeny  for  a  longer  time  and  with  greater  analytical  ability 
than  any  previous  investigator  had  done.  The  methods  and  re- 
sults of  his  work  are  so  well  known  through  the  writings  of  Bate- 
son,  Punnett,  and  many  others  that  it  is  unnecessary  to  dwell  at 
length  upon  them  here.  In  brief  Mendel's  method  consisted  in 
crossing  two  forms  having  distinct  characters,  and  then  in  count- 
ing the  number  of  offspring  in  successive  generations  showing  one 
or  the  other  of  these  characters. 

Mendel's  Experiments  on  Peas. — During  the  eight  years  pre- 
ceding the  publication  of  his  paper  in  1866  Mendel  hybridized 
some  twenty-two  varieties  of  garden  peas.  This  group  of  plants 
was  chosen  because  the  different  varieties  could  be  cross-fertilized 
or  self-fertilized  and  were  easily  protected  from  the  influence  of 
foreign  pollen;  because  the  hybrids  and  their  offspring  remained 
fertile  through  successive  generations;  and  because  the  different 
varieties  are  distinguished  by  constant  differentiating  characters. 
Mendel  devoted  his  attention  to  seven  of  these  contrasting  char- 
acters, which  he  followed  through  several  generations  of  hybrids, 
viz., 

1 i )  Differences  in  the  form  of  the  ripe  seeds,  whether  round  or 
wrinkled. 

(2)  Differences  in  the  color  of  the  food  material  within  the 
seeds,  whether  pale  yellow,  orange  or  green. 

(3)  Differences  in  the  color  of  the  seed  coats  (and  in  some 
oases  of  the  flowers  also),  whether  white,  gray,  gray  brown, 
leather  brown,  with  or  without  violet  spots. 

(4)  Differences  in  the  form  of  the  ripe  pods,  whether  simply 
inflated  or  constricted  between  the  seeds. 

(5)  Differences  in  the  color  of  the  unripe  pods,  whether  light 
to  dark  green,  or  vividly  yellow. 


84  Heredity  and  Environment 

(6)  Differences  in  the  positions  of  the  flowers,  whether  axial, 
that  is,  distributed  along  the  stem,  or  terminal,  that  is,  bunched  at 
the  top  of  the  stem. 
.  (7)  Differences  in  the  length  of  the  stem,  whether  tall  or  short. 

i.  Results  of  Crossing  Individuals  with  one  Pair  of  Contrast- 
ing Characters. — Having  determined  that  these  characters  were 
constant  for  certain  varieties  Mendel  then  proceeded  to  cross 
one  variety  with  another,  by  carefully  removing  the  unripe  sta- 
mens, with  their  pollen,  from  the  flowers  of  one  variety  and  dust- 
ing upon  the  stigmas  of  such  flowers  the  pollen  of  a  different 
variety.  In  this  way  he  crossed  varieties  of  peas  which  differed 
from  each  other  in  some  one  of  the  characters  mentioned  above, 
and  then  studied  the  offspring  of  several  successive  generations 
with  respect  to  this  character. 

Dominant  and  Recessive  Characters. — In  every  case  he  discov- 
ered that  the  plants  that  developed  from  such  a  cross  showed  only 
one  of  the  two  contrasting  characters  of  the  parent  plants,  i.e.}  all 
were  round-seeded,  yellow-seeded,  or  tall,  etc.,  although  one  of  the 
parents  had  wrinkled  seeds,  green  seeds,  or  short  stem,  etc.  "Those 
characters  which  are  transmitted  entire  or  almost  unchanged  in 
the  hybridization  are  termed  dominant,  and  those  which  become 
latent  in  the  process,  recessive!' 

Ratio  of  Dominants  to  Recessives. — These  hybrids*  when  self- 
fertilized  gave  rise  to  a  second  filial  generation  of  individuals 
some  of  which  showed  the  dominant  character  and  others  the  re- 
cessive, the  relative  numbers  of  the  two  being  approximately 
three  to  one.  Thus  the  hybrids  produced  by  crossing  yellow- 

*  Bateson  introduced  the  term  "homo-zygote"  for  pure-bred  individuals 
resulting  from  the  union  of  gametes  which  are  hereditarily  similar,  and 
"hetero-zygote"  for  hybrids  resulting  from  the  union  of  hereditarily  dis- 
similar gametes.  The  gametes  formed  from  a  homo-zygote  are  all  of  the 
same  hereditary  type,  those  formed  from  a  hetero-zygote  are  of  two  dif- 
ferent types  for  every  unit  difference  of  the  parents.  The  members  of  a 
pair  of  contrasting  characters  are  called  "allelomorphs";  eacsh  member 
of  such  a  pair  is  "allelomorphic"  to  the  other  member. 


Phenomena  of  Inheritance 


seeded  and  green-seeded  peas  yielded  when  self-fertilized  6,022 
yellow  seeds  and  2,001  green  seeds,  or  very  nearly  three  yellow  to 
one  green  (Fig.  26).  The  hybrids  produced  by  crossing  round 
and  wrinkled  seeded  varieties  yielded  in  the  second  filial  genera- 
tion 5,474  round  and  1,850  wrinkled  seeds,  or  approximately  three 
round  to  one  wrinkled  (Fig.  30).  The  hybrids  from  tall-stemmed 
and  short-stemmed  parents  produced  in  the  second  filial  genera- 
oo  •  • 


Parents 


FIG.  26.  DIAGRAM  SHOWING  THE  RESULTS  OF  CROSSING  YELLOW-SEEDED 
(LIGHTER  COLORED)  AND  GREEN-SEEDED  (DARKER  COLORED)  PEAS.  (From 
Morgan  after  Thompson.) 


86 


Heredity  and  Environment 


tion  787  long-stemmed  and  277  short-stemmed  plants,  or  again 
approximately  three  tall  to  one  short.  And  in  every  other  case 
Mendel  found  that  the  ratio  of  dominants  to  recessives  in  the 
second  filial  generation  was  approximately  three  to  one. 

"Extracted"  Dominants  or  Recessives. — These  recessives 
derived  from  hybrid  parents  are  pure  and  are  known  as  "ex- 
tracted" recessives;  when  self-fertilized  they  produce  only  re- 
cessives. One-third  of  the  dominants  are  also  pure  homozy- 
gotes,  or  "extracted"  dominants,  and  when  self-fertilized  pro- 
duce only  pure  dominants.  On  the  other  hand  two-thirds  of 
the  dominants  are  heterozygotes  and  when  self-fertilized  give 
rise  in  the  next  generation  to  pure  dominants,  dominant-reces- 
sives  and  pure  recessives  in  the  proportion  of  1:2:1. 

These  general  results  are  summarized  in  the  accompanying 
diagram  (Fig.  27)  in  which  dominant  characters  are  indicated 
by  the  letter  D,  recessive  characters  by  R,  and  dominant-re- 
cessives,  with  the  recessive  character  unexpressed,  by  D  (R)  ; 
while  DD  or  RR  indicate  extracted  dominants  or  recessives,  that 
is,  pure  dominants  or  recessives  which  have  separated  out  from 
dominant-recessives,  D  (R).  The  parental  generation  is  usually 
indicated  by  the  letter  P,  and  the  successive  filial  generations  by 
Flf  F2,  F3,  etc. 


Parent    Generation 

r*      • 


Homozygotea 
Helerozygotes 


Fa    • 


FIG.  27.  DIAGRAM  SHOWING  RESULTS  OF  MENDELIAN  SPLITTING  where 
the  parents  are  pure  dominants  and  pure  recessives  (homozygotes).  All 
pure  dominants  are  represented  by  black  circles,  all  pure  recessives  by 
white  ones,  while  dominant-recessives  (heterozygotes)  are  represented 
by  circles  half  white  and  half  black.  Successive  generations  are  marked 
F,,  F2,  Flf  etc. 


Phenomena  of  Inheritance  87 

Incomplete  Dominance. — In  the  case  of  the  peas  studied  by 
Mendel  the  hybrids  of  the  Ft  generation  show  only  the  domi- 
nant character,  the  contrasted  recessive  character  being  present 
but  not  expressed.  However  in  certain  cases  it  has  been  found 
that  the  hybrids  differ  from  either  parent  and  in  successive  gener- 
ations split  up  into  both  parental  types  and  into  the  hybrid  type ; 
thus  Correns  found  that  when  a  white-flowered  variety  of  Mira- 
bilis,  the  "four  o'clock,"  was  crossed  with  a  red-flowered  variety  all 
of  the  hybrids  in  the  FA  generation  had  pink  flowers  and  from 
these  in  the  F2  generation  there  came  white-flowered,  pink-flow- 


FIG.  28.  RESULTS  OF  CROSSING  WHITE-FLOWERED  AND  RED-FLOWERED 
RACES  OF  Mirabilis  Jalapa  ("four  o'clocks")  giving  a  pink  hybrid  in  Flf 
which  when  inbred  gives  in  F0  i  white,  2  pink,  i  red.  The  gametes  bear- 
ing white  or  red  are  indicated  by  white  or  black  circles.  (From  Woodruff, 
after  Correns.) 


88  Heredity  and  Environment 

ered  and  red-flowered  forms  in  the  proportion  of  I  white :  2 
pink:  I  red,  as  shown  in  Fig.  28.  This  is  a  better  illustration  of 
Mendel's  principle  of  splitting  than  is  offered  by  the  peas,  since 
in  this  case  the  heterozygotes  D(R)  are  always  distinguishable 
from  the  pure  dominants  DD. 

Results  in  Later  Generations. — In  the  F2  generation  and  in  all 
subsequent  ones  the  pure  dominants  and  the  pure  recessives  al- 
ways breed  true  when  self-fertilized,  whereas  the  heterozygotes 
continue  to  split  up  in  each  successive  generation  into  pure  domi- 
nants, heterozygotes  and  pure  recessive  in  the  proportion  of 
1:2:  i.  The  result  of  this  is  that  with  continued  self-fertiliza- 
tion the  relative  number  of  dominants  and  recessives  increases 
in  successive  generations,  whereas  the  relative  number  of  hetero- 
zygotes decreases,  and  in  a  few  generations  a  hybrid  race  will 
revert  in  large  part  to  its  parental  types  if  continued  hybridiza- 
tion is  prevented.  On  the  other  hand  there  is  no  tendency  for 
the  relative  number  of  dominants  to  increase  and  of  recessives 
to  decrease  in  successive  generations;  an  equal  number  of  pure 
dominants  and  pure  recessives  is  produced  in  each  generation 
(Fig.  27). 

"Pwity"  of  Germ  Cells.— With  remarkable  insight  Mendel 
recognized  that  the  real  explanation  of  the  splitting  of  pure 
recessives  and  pure  dominants  from  hybrid  parents  must  be 
found  in  the  composition  of  the  male  and  female  sex  cells- 
Since  such  extracted  dominants  and  recessives  breed  true,  just 
as  pure  species  do,  it  must  be  that  their  germ  cells  are  pure. 
In  the  cross  between  pure  races  of  white-flowered  and  red- 
flowered  Mirabilis  the  germ  cells  which  unite  in  fertilization 
must  be  pure  with  respect  to  white  and  red,  though  the 
individual  which  develops  from  this  cross  is  a  pink  hy- 
brid. But  the  fact  that  one-quarter  of  the  progeny  of  this  hy- 
brid are  pure  white,  and  another  quarter  pure  red,  and  that  these 
thereafter  breed  true,  proves  that  the  hybrid  produces  germ  cells 
which  are  pure  with  respect  to  red  and  white.  Furthermore  the 
fact  that  one-half  the  progeny  of  this  hybrid  are  themselves  hybrid 


Phenomena  of  Inheritance  89 

* 

may  be  explained  by  assuming  that  they  were  produced  by  the 
union  of  germ  cells  carrying  pure  white  and  pure  red,  as  in  the 
parental  generation. 

Mendel  therefore  concluded  that  individual  germ  cells  are  al- 
ways pure  with  respect  to  any  pair  of  contrasting  characters, 
even  though  those  germ  cells  have  come  from  hybrids  in  which 
the  contrasting  characters  are  mixed.  A  single  germ  cell  can 
carry  the  factor  for  red  flowers  or  white  flowers,  for  green  seeds 
or  yellow  seeds,  for  tall  stem  or  short  stem,  etc.,  but  not  for  both 
pairs  of  these  contrasting  characters.  The  hybrids  formed  by 
crossing  white  and  red  "four  o'clocks"  carry  the  factors  for  both 
white  and  red,  but  the  individual  germ  cells  formed  by  such  a 
hybrid  carry  the  factors  for  white  or  red,  but  not  for  both  ;  these 
factors  segregate  or  separate  in  the  formation  of  the  germ  cells 
so  that  one-half  of  all  the  germ  cells  formed  carry  the  factor  for 
white  and  the  other  half  that  for  red. 

This  is  the  most  important  part  of  Mendel's  Law,  —  the  central 
doctrine  from  which  all  other  conclusions  of  his  radiate.  It  ex- 
plains not  only  the  segregation  of  dominant  and  recessive  charac- 
ters from  a  hybrid  in  which  both  are  present,  but  also  the  relative 
numbers  of  pure  dominants,  pure  recessives  and  heterozygotes 
in  each  generation.  For  if  all  germ  cells  are  pure  with  re- 
spect to  any  particular  character  the  hybrid  offspring  of  any 
two  parents  with  contrasting  characters  will  produce  in  equal 
numbers  two  classes  of  germ  cells,  one  bearing  the  dominant  and 
the  other  the  recessive  factor,  and  the  chance  combination  of  these 
two  classes  of  male  and  female  gametes  will  yield  on  the  average 
one  union  of  dominant  with  dominant,  two  unions  of  dominant 
with  recessive  and  one  union  of  recessive  with  recessive,  thus 
producing  the  typical  Mendelian  ratio,  iDD  :  2D(R)  :  iRR,  as 
shown  in  the  accompanying  diagram  and  in  Fig.  29  b. 

9  germ  cells  D 


$  germ  cells  D 
Possible  combinations         I  DD  :  2D(R)  :  I  RR. 


Heredity  and  Environment 


Other  Mendelian  Ratios. — When  a  pure  dominant  is  crossed 
with  a  hybrid  dominant-recessive  (Fig.  29  r)  all  of  the  offspring 
show  the  dominant  character,  though  one-half  are  pure  dominants 
and  the  other  half  dominant-recessives.  Thus  if  a  pure  round- 
seeded  variety  of  pea  is  crossed  with  a  hybrid  between  a  round- 
seeded  and  a  wrinkled-seeded  one,  all  the  progeny  are  round- 
seeded,  though  one-half  of  them  carry  the  factor  for  wrinkled 
seed;  this  may  be  graphically  represented  as  follows,  R  repre- 
senting the  factor  for  round  seed  and  W  that  for  wrinkled  seed : 

$  germ  cells  R^      .R 

1X1 

a  germ  cells  R       ^W 


Possible  combinations 


p      a 


2RR: 


Fi 


FIG.  29.  DIAGRAM  OF  MENDELIAN  INHERITANCE,  in  which  the  individual 
is  represented  by  the  large  circle,  the  germ  cells  by  the  small  ones,  domi- 
nants being  shaded  and  recessives  white,  a,  Pure  dominant  X  pure  re- 
cessive —  (yields)  all  dominant-recessives ;  bf  Dominant-recessive  X  domi- 
nant-recessive =  i  pure  dominant :  2  dominant-recessives  :  I  pure  recessive ; 
c,  Dominant-recessive  X  pure  dominant  =  2  pure  dominant :  2  dominant- 
recessive;  d,  Dominant-recessive  X  pure  recessive  =  2  dominant-reces- 
sive :  2  pure  recessive. 


Phenomena  of  Inheritance  gi 

In  subsequent  generations  the  progeny  of  the  pure  round  (RR) 
breed  true  and  produce  only  round-seeded  peas,  whereas  the  pro- 
geny of  the  hybrid  round-wrinkled  (RW)  split  up  into  pure 
round,  hybrid  round-wrinkled,  and  pure  wrinkled  in  the  regular 
Mendelian  ratio  of  i  RR :  2  R(lfi")  :  I  WW  (Fig.  30). 

When  a  pure  recessive  is  crossed  with  a  hybrid  dominant- 
recessive  (Fig.  29,  d)  another  typical  ratio  results.  Thus  if  a 
wrinkled-seeded  variety  of  pea  is  crossed  with  a  hybrid  between 
a  round-seeded  and  wrinkled-seeded  one,  round-seeded  and 
wrinkled-seeded  peas  are  produced  in  the  proportion  of  I :  I  This 
is  due  to  the  fact  that  the  hybrid  produces  two  kinds  of  germ 
cells,  the  pure-bred  but  one,  and  the  possible  combinations  of 
these  are  as  follows: 

$  germ  cells  W.        W 

1X1 

$  germ  cells  A          W 
Possible  combinations  2  R  ( W)  :      2  WW. 

This  ratio  of  2 : 2  or  I :  i  is  approximately  the  ratio  of  the  two 
sexes  in  many  animals  and  plants,  and  there  is  good  reason  to  be- 
lieve that  sex  is  a  Mendelian  character  of  this  sort,  in  which  one 
parent  is  heterozygous  for  sex  and  the  other  homozygous  (See 
p.  167). 

2.  Results  of  Crossings  where  there  is  more  than  one  Pair  of 
Contrasting  Characters. — It  rarely  happens  that  two  individuals 
differ  in  a  single  character  only;  more  frequently  they  differ  in 
many  characters,  and  this  leads  to  a  great  increase  in  the  number 
of  types  of  offspring  in  the  F2  generation.  But  however  many 
pairs  of  contrasting  characters  the  parents  may  show  each  pair 
may  be  considered  by  itself  as  if  it  were  the  only  contrasting  pair, 
and  when  this  is  done  all  the  offspring  may  be  classified  accord- 
ing to  the  regular  Mendelian  formula  given  above. 

When  the  parents  differ  in  one  unit  character  only,  the  offspring 
formed  by  their  crossing  are  called  mono-hybrids,  when  there  are 


Heredity  and  Environment 


two  contrasting  characters  in  the  parents  the  offspring  are  di-hy- 
brids,  when  three,  tri-hybrids,  and  when  the  parents  differ  in  more 
than  three  characters  the  offspring  are  called  poly-hybrids.  There 
are  certainly  few  cases  in  which  parents  actually  differ  in  only  a 
single  character,  but  since  each  contrasting  character  may  be 
dealt  with  separately,  as  if  it  were  the  only  one,  and  since  the 
number  of  types  of  offspring  increases  greatly  when  more  than 
one  or  two  characters  are  considered  at  the  same  time,  it  is  cus- 
tomary to  deal  simultaneously  with  only  one  or  two  characters 
of  hybrids,  even  though  the  parents  may  have  differed  in  many 
characters.  The  different  types  of  developed  organisms  are  called 
by  Johannsen  "phenotypes"  whereas  the  different  hereditary 
types  whether  patent  or  latent  are  called  "genotypes-" 
Dihybrids. — When  two  or  more  contrasting  characters  of  the 


w 


FlG.   3O.      MONOHYBRID  DIAGRAM    SHOWING   RESULTS    OF    CROSSING   ROUND- 

(/?)  SEEDED  WITH  WRINKLED-  (W)  SEEDED  PEAS.  Large  circles  represent 
zygotes,  small  ones,  or  single  letters,  gametes.  In  Fl  all  individuals  are 
round  but  contain  round  and  wrinkled  gametes.  In  F2  the  $  gametes 
are  placed  above  the  square,  the  $  ones  to  the  left,  and  the  possible  com- 
binations of  $  and  9  gametes  are  shown  in  the  small  squares,  the  relative 
number  of  different  genotypes  being  I  RR  :  2  R(W)  :i  WW. 


Phenomena  of  Inheritance 


93 


parents  are  followed  to  the  F2  generation  all  possible  permutations 
of  these  characters  occur,  thus  giving  rise  to  a  larger  number  of 
types  of  individuals  than  when  a  single  pair  of  characters  is 
concerned.  When  there  is  only  one  pair  of  contrasting  characters 
there  are  three  genotypes  and  usually  but  two  phenotypes  in  the 


GW 


FIG.  31.  DIHYBRID  DIAGRAM  SHOWING  RESULTS  OF  CROSSING  PEAS  HAV- 
ING YELLOW  ROUND  (YR)  SEEDS  WITH  OTHERS  HAVING  GREEN  WRINKLED 
(GW)  ONES.  The  hybrids  of  the  first  filial  generation  (FJ  are  all  yellow 
and  round  since  these  characters  are  dominant  while  green  and  wrinkled 
are  recessive,  YR(GW}.  Four  types  of  germ  cells  are  formed  by  such  a 
hybrid,  viz.,  YR,  YW ,  GR,  GW,  and  the  16  possible  combinations  of  these 
$  and  $  gametes  are  shown  in  the  small  squares.  Of  these  16  combina- 
tions 7  contain  the  same  letters  (factors)  so  that  there  are  only  9  differ- 
ent genotypes,  and  since  recessive  characters  do  not  appear  when  mated 
with  dominant  ones  these  9  genotypes  produce  only  4  phenotypes  in  the 
following  relative  numbers:  9  YR :  3  YW ':  3  GR :  I  GW.  There  is  I  pure 
dominant  (upper  left  corner),  I  pure  recessive  (lower  right  corner)  ; 
4  homozygotes  in  the  diagonal  line  between  these  corners,  and  12  heterozy- 
gotes. 


94  Heredity  and  Environment 

F2  generation,  viz.,  dominants  and  recessives  in  the  ratio  of  3 :  i 
(Fig.  30)  ;  where  there  are  two  pairs  of  contrasting  characters 
in  the  parents  there  are  nine  genotypes  (32)  and  usually  four 
phenotypes  in  the  F2  generation  in  the  ratio  of  (3:  i )2  =  9 :  3  :  3  :  i. 
Thus  when  Mendel  crossed  a  variety  of  peas  bearing  round  and 
yellow  seeds  with  another  variety  having  wrinkled  and  green 
seeds  all  the  offspring  of  the  F^  generation  bore  round  and  yellow 
seeds,  round  being  dominant  to  wrinkled,  and  yellow  to  green. 
But  the  plants  raised  from  these  seeds,  when  self-fertilized, 
yielded  seeds  of  four  types,  yellow  and  round  (YR),  yellow  and 
wrinkled  (YW),  green  and  round  (GR),  and  green  and  wrinkled 
(GW)  in  the  proportion  of  9:  3 :  3 :  i  as  shown  in  Fig.  31. 

In  this  case  also  this  ratio  may  be  explained  by  assuming  that 
the  germ  cells  are  pure  with  respect  to  each  of  the  contrasting 
characters,  round  or  wrinkled,  yellow  or  green,  and  therefore  any 
combination  of  these  may  occur  in  a  germ  cell  except  the  com- 
binations RW  and  YG.  Accordingly  there  are  four  possible 
combinations  of  these  characters  in  both  male  and  female  cells 
as  follows : 

Y  G 

|  X  |      i.e.  YR,  YW,  GRf  GW. 

R  W 

Each  of  these  four  kinds  of  male  cells  may  fertilize  any  one  of  the 
same  four  kinds  of  female  cells,  thus  giving  rise  to  sixteen  com- 
binations, as  shown  in  Fig.  31.  The  dominant  characters  are  in 
this  case  round  and  yellow,  and  only  when  one  of  these  is  absent 
can  its  contrasting  character,  wrinkled  or  green,  develop.  Ac- 
cordingly the  sixteen  possible  combinations  yield  seeds  of  four 
different  appearances  and  in  the  following  proportions :  9  YR : 
3  GR :  3  YW  \  i  GW.  Only  one  individual  in  each  of  these  four 
classes  is  pure  (homozygous)  and  continues  to  breed  true  in 
successive  generations;  in  Fig.  31  these  are  found  in  the  diagonal 
from  the  upper  left  to  the  lower  right  corner.  All  these  in- 


Phenomena  of  Inheritance  95 

dividuals  are  heterozygous  and  show  Mendelian  splitting  in  the 
next  generation. 

Trihybrids. — When  parents  differ  in  three  contrasting  charac- 
ters there  are  twenty-seven  genotypes  (33)  and  eight  phenotypes 
(23)  in  the  F2  generation  in  the  proportion  of  (3:  i)3  =  27:9: 
9:9:3:3:3:  r-  Thus  if  a  pea  with  round  (R)  and  yellow  ( F) 
seeds  and  with  tall  (T)  stem  is  crossed  with  one  having  wrinkled 
(IV)  and  green  (G)  seeds  and  dwarf  (D)  stem  all  the  progeny 
of  the  F!  generation  have  round  and  yellow  seeds  and  tall  stem, 
R,  Y,  and  T  being  dominant  over  W,  G,  and  D.  In  the  F2  gener- 
ation there  are  64  possible  combinations  (27  genotypes)  of  these 
six  characters  (Fig.  32)  ;  but  since  a  recessive  character  does  not 
develop  if  its  contrasting  dominant  character  is  present  there  are 
only  eight  phenotypes  which  come  to  expression  and  in  the 
following  ratios:  27  RYT:g  RYD:g  RGT:g  WYT:$  RGD: 
3  IVYD  :  3  WGT:  i  WGD.  Of  these  sixty- four  combinations  only 
eight  are  homozygous  and  breed  true  (those  lying  in  the  diagonal 
between  upper  left  and  lower  right  corners  in  Fig.  32),  while  only 
one  is  a  pure  dominant  and  one  a  pure  recessive  (the  ones  in  the 
upper  left  and  lower  right  corners  of  Fig.  32). 

3.  Inheritance  Formulae. — Mendel  represented  the  hereditary 
constitution  of  the  plants  used  in  his  experiments  by  letters  em- 
ployed as  symbols,  dominant  characters  being  represented  by  capi- 
tals and  recessives  by  small  letters.  The  seven  contrasting  char- 
acters of  his  peas  could  be  represented  as  follows : 

Seeds,  round  (A),  or  wrinkled  (a) ;  yellow  (B),  or  green  (b) ; 
with  gray  seed  coats  (C),  or  white  seed  coats  (c). 

Pods,  green  (D),  or  yellow  (rf) ;  inflated  (E),  or  constricted 

(0- 

Habit,  tall  (F),  or  dwarf  (/). 

Flowers,  axial  (G),  or  terminal  (g). 

It  is  possible  for  one  plant  to  have  all  of  these  dominant  char- 
acters or  all  of  the  recessive  ones,  or  part  of  one  kind  and  part  of 
the  other.  The  gametic  formula  of  a  plant  having  all  seven  of 


Heredity  and  Environment 
WGD 


F, 


RYT       RYD        RGT       ROD       WYT      WYD       WGT       WGD 


FIG.  32.  TRIHYBRID  DIAGRAM  SHOWING  RESULTS  OF  CROSSING  PEAS  HAV- 
ING ROUND  YELLOW  SEEDS  AND  TALL  STEM  (RYT)  WITH  PEAS  HAVING 
WRINKLED  GREEN  SEEDS  AND  DWARF  STEM  (WGD).  Eight  types  of  germ 
cells  are  formed  by  the  F1  hybrid,  as  shown  in  the  $  gametes  above  the 
square  and  the  $  ones  to  the  left  of  it,  and  the  possible  combinations 
of  these  $  and  $  gametes  are  shown  in  the  64  small  squares  of  which 
only  I  is  pure  dominant  (upper  left  corner),  I  pure  recessive  (lower' 
right  corner)  and  8  homozygotes  (in  diagonal  line  between  these  corners). 
There  are  27  different  genotypes,  all  combinations  below  this  diagonal  be- 
ing homologous  with  the  corresponding  ones  above  and  all  in  the  other 
diagonal  being  of  the  same  genotype,  while  12  other  combinations  on 
each  side  of  the  first  diagonal  constitute  only  6  genotypes.  There  are  8 
phenotypes,  resembling  the  8  homozygotes,  and  their  relative  numbers 
are  27  RYT : 9  RYD:  9  RGT \  9  IVY T :  3  ROD :  3  WYD -.3  WGT:  i  WGD. 


Phenomena  of  Inheritance  97 

the  dominant  characters  is  ABCDEFG ;  of  one  having  all  of  the 
recessive  characters  abcdefg.  When  two  such  plants  are  crossed 
the  zygotic  formula  of  the  hybrid  is  AaBbCcDdEcFfGg,  and 
since  the  dominant  and  recessive  characters  (or  rather  determin- 
ers of  characters)  represented  by  these  seven  pairs  of  letters 
separate  in  the  formation  of  the  gametes;  and  since  each  separate 
determiner  may  be  associated  with  either  member  of  the  six 
other  pairs,  the  number  of  possible  combinations  of  these  deter- 
miners in  the  gametes  is  (2)*  or  128.  That  is,  in  this  case  128 
kinds  of  germ  cells  may  be  produced,  each  having  a  different  in- 
heritance formula ;  and  since  each  of  these  128  kinds  of  male  germ 
cells  may  unite  with  any  one  of  the  128  kinds  of  female  germ  cells 
the  number  of  combinations  of  these  characters  which  are  pos- 
sible in  the  F2  generation  is  (i28)2  or  16,384,  while  the  number 
of  different  genotypes  is  (3)7  or  2187.  Every  one  of  these  more 
than  two  thousand  genotypes  may  be  represented  by  various  com- 
binations of  the  letters  ABCDEFG  and  abcdefg. 

When  many  characters  are  concerned  it  is  difficult  to  remember 
what  each  letter  stands  for,  and  consequently  it  is  customary  in 
such  cases  to  designate  characters  by  the  initial  letter  in  the  name 
of  that  character.  By  this  form  of  shorthand  one  can  show  in  a 
graphic  way  the  possible  segregations  and  combinations  of  heredi- 
tary units  in  gametes  and  zygotes  through  successive  generations, 
and  as  a  result  many  modern  works  on  Mendelian  inheritance  look 
like  pages  of  algebraic  formulae. 

4.  Presence  and  Absence  Hypothesis. — Mendel  spoke  of  the 
presence  of  contrasting . or  differentiating  characters  in  the  plants 
which  he  crossed,  such  as  round  or  wrinkled  seeds,  tall  or  short 
stems,  etc.  Many  others  have  regarded  these  contrasting  charac- 
ters as  due  to  the  presence  or  absence  of  single  factors:  thus 
round  seeds  are  due  to  the  presence  of  a  factor  for  roundness 
(A)  while  wrinkled  seeds  were  said  to  be  clue  to  the  absence  of 
that  factor  (a).  Round  seeds  were  spoken  of  as  wrinkled  seeds 
plus  the  factor  for  roundness.  But  it  is  practically  certain  that 


98  Heredity  and  Environtnent 

recessive  characters  are  not  due  to  the  absence  of  factors  for 
dominant  characters;  there  are  many  genetical  and  philosophical 
objections  to  such  a  view,  which  leads  logically  to  some  strange 
conclusions,  such  as  Bateson's  speculations  on  evolution  (p.  282). 
Morgan  and  his  associates  have  found  that  a  given  dominant 
character  may  have  several  different  kinds  of  recessive  contrast- 
ing characters  or  allelomorphs ;  thus  the  dominant  eye  color  of  the 
wild  pomace  fly,  Drosophila  melanogaster,  is  red,  but,  instead  of 
a  single  contrasting  recessive  character,  eleven  have  been  found, 
viz.,  apricot,  blood,  buff,  cherry,  coral,  ecru,  eosin,  ivory,  tinged, 
wine,  and  white.  Such  a  condition  is  known  as  "multiple  allelo- 
morphism."  If  the  red  color  is  due  to  the  presence  of  a  certain 
factor,  all  these  other  allelomorphic  colors  cannot  be  due  to  its 
absence,  since,  there  can  be  only  one  kind  of  absence.  Each  of 
these  recessive  colors  must  be  due  to  the  presence  of  a  differentia) 
factor.  Therefore  the  presence-absence  hypothesis  must  be  aban- 
doned. 

When  both  gametes  carry  similar  dominant  factors  the  zygote 
has  a  "double  dose"  of  such  factors  and  is  said  to  be  duplex  \ 
when  only  one  of  the  gametes  carries  such  a  factor  the  zygote 
has  a  "single  dose"  and  is  simplex,  when  neither  gamete  carries 
a  positive  factor  or  factors,  the  zygote  receives  only  negative  fac- 
tors and  is  said  to  be  nulliplex.  Thus  the  union  of  gametes 
AB  (  9  )  and  AB  ( $  )  yields  zygote  AABB,  which  is  duplex  in 
constitution ;  gametes  Ab  (  $  )  and  aB  (  $  )  yield  zygote  AaBb, 
which  is  simplex ;  gametes  ab  (  $  )  and  ab  (  $  )  yield  zygote  aabb, 
which  is  nulliplex. 

In  some  instances  a  character  comes  to  full  expression  only 
when  it  is  derived  from  both  parents,  thaj:  is,  when  it  is  duplex; 
if  derived  from  one  parent  only,  that  is,  if  simplex,  it  is  diluted 
in  appearance  and  is  intermediate  between  the  two  parents.  For 
example,  when  white-flowered  "four  o'clocks"  which  are  nulli- 
plex are  crossed  with  red-flowered  ones  which  are  duplex 
the  progeny,  which  are  simplex,  bear  pink  flowers;  in  this  case 
red  flowers  are  produced  only  when  the  factor  for  red  is  derived 


Phenomena  of  Inheritance  99 

from  both  parents,  pink  flowers  when  it  is  derived  from  one  parent, 
white  flowers  when  it  is  derived  from  neither  parent  (Fig.  28). 

5.  Summary  of  Mendelian  Principles. — Since  the  rediscovery 
in  1900  of  Mendel's  work  many  investigators  have  carried  out 
similar  experiments  on  many  species  of  animals  and  plants  and 
have  greatly  extended  our  knowledge  of  the  principles  of  inheri- 
tance discovered  by  Mendel,  but  in  the  main  Mendel's  conclu- 
sions have  been  confirmed  again  and  again,  'so  that  there  is  no 
doubt  that  they  constitute  an  important  rule  of  inheritance  among 
all  sexually  produced  organisms. 

In  brief  the  "Mendelian  Law  of  Alternative  Inheritance"  or  of 
hereditary  "splitting"  consists  of  the  following  principles: 

(a)  The  Principle  of  Unit  Characters. — The  heritage  of  an 
organism  may  be  analyzed  into  a  number  of  characters  which  are 
inherited  as  a  whole  and  are  not  further  divisible;  these  are  the 
so-called  "unit  characters"  (deVries). 

(b)  The  Principle  of  Dominance. — When   contrasting  unit 
characters  are  present  in  the  parents  they  do  not  as  a  rule  blend 
in  the  offspring,  but  one  is  dominant  and  usually  appears  fully 
developed,  while  the  other  is  recessive  and  temporarily  drops  out 
of  sight. 

(c)  The  Principle  of  Segregation. — Every  individual  germ  cell 
is  "pure"  with  respect  to  any  given  unit  character,  even  though 
it  come  from  an  "impure"  or  hybrid  parent.     In  the  germ  cells 
of  hybrids  there  is  a  separation  of  the  determiners  of  contrasting 
characters  so  that  different  kinds  of  germ  cells  are  produced, 
each  of  which  is  pure  with  regard  to  any  given  unit  character. 
This  is  the  principle  of  segregation  of  unit  characters,  or  of  the 
"purity"  of  the  germ  cells.    Every  sexually  produced  individual 
is  a  double  being,  double  in  every  cell,  one-half  of  its  determiners 
having  been  derived  from  the  male  and  the  other  half  from  the 
female  sex  cell.     This  double  set  of  determiners  again  becomes 
single  in  the  formation  of  the  germ  cells  only  once  more  to  be- 
come double  when  the  germ  cells  unite  in  fertilization. 


ioo  Heredity  and  Environment 

II.    MODIFICATIONS  AND  EXTENSIONS  OF  MENDELIAN 
PRINCIPLES 

It  is  a  common  experience  that  natural  phenomena  are  found 
to  be  more  complex  the  more  thoroughly  they  are  investigated. 
Nature  is  always  greater  than  our  theories,  and  with  few  excep- 
tions hypotheses  which  were  satisfactory  at  one  stage  of  knowl- 
edge have  to  be  extended,  modified  or  abandoned  as  knowledge 
increases.  This  observation  is  well  illustrated  in  the  case  of  the 
Mendelian  theory.  The  principles  proposed  by  Mendel  were  rela- 
tively simple,  but  in  attempting  to  apply  them  to  the  many  phe- 
nomena of  inheritance  now  known  it  has  become  necessary  to 
modify  or  extend  them  in  many  ways.  And  yet  the  general  and 
fundamental  truth  of  these  principles  has  been  established  in  a 
surprisingly  large  number  of  cases,  and  they  have  been  extended 
to  forms  of  inheritance  where  at  first  it  was  supposed  that  they 
could  not  apply. 

I.  The  Principle  of  Unit  Characters  and  of  Inheritance  Fac- 
tors.— There  has  been  much  criticism  on  the  part  of  some  biolo- 
gists of  the  principle  of  unit  characters.  It  is  said  that  unit  char- 
acters cannot  be  independent  and  discrete  things;  the  organism 
itself  is  a  unity  and  every  one  of  its  parts,  every  one  of  its  char- 
acters, must  influence  more  or  less  every  other  part  and  every 
other  character.  Certainly  unit  characters  cannot  be  absolutely 
independent  of  one  another ;  the  various  parts  and  organs  of  the 
body,  and  even  the  organism  as  a  whole,  are  not  absolutely  inde- 
pendent, and  yet  there  are  varying  degrees  of  independence  in 
organisms,  organs,  cells,  parts  of  cells,  hereditary  units  and  char- 
acters which  make  it  possible  for  purposes  of  analysis  to  deal 
with  these  things  as  if  they  were  really  independent  though  we 
know  they  are  not.  But  the  most  serious  objection  to  the  doctrine 
of  unit  characters  is  not  against  their  independence  but  against 
their  unity.  Every  character  is  complex,  many  factors  enter  into 
its  development,  and  since  the  combination  of  these  factors  is 
variable  the  character  itself  cannot  be  constant.  Strictly  speaking. 


Phenomena  of  Inheritance  toi 

characters  are  not  units,  and  while  the  conception  of  "unit  char- 
acters" has  served  a  useful  purpose  it  cannot  any  longer  be  re- 
garded as  wholly  accurate. 

Inheritance  Factors  are  Differential  Causes. — Of  course  char- 
acters of  adult  individuals  do  not  exist  as  such  in  germ  cells,  but 
there  is  no  escape  from  the  conclusion  thaUin  the  case  of  inherited 
differences  between  mature  organisms  there  must  haVe  been  dif- 
ferences in  the  constitution  of  the  germ  cells  from  which  they 
developed.  For  every  inherited  character  there  must  have  been  a 
germinal  cause  in  the  fertilized  egg.  This  germinal  cause,  what- 
ever it  may  be,  is  often  spoken  of  as  a  determiner  of  a  character. 
But  the  character  in  question  is  not  to  be  thought  of  as  the  result 
of  a  single  cause  nor  as  the  product  of  the  development  of  a 
single  determiner;  undoubtedly  many  causes  are  involved  in  the 
development  of  every  character,  but  the  differential  cause  or  com- 
bination of  causes  is  that  which  is  peculiar  to  the  development  of 
each  particular  character.  Of  course  Mendelian  factors  are  not 
the  only  factors  of  development  but  merely  the  differential  factors 
which  cause,  for  example,  one  guinea-pig  to  be  white  and  its  bro- 
ther to  be  black.  Very  many  factors  are  involved  in  the  produc- 
tion of  white  or  black  color  but  there  is  at  least  one  differential 
factor  for  every  unit  character  and  this  alone  is  the  Mendelian 
factor. 

Factors  Are  Not  Undeveloped  Characters. — Again  it  is  not 
necessary  to  suppose  that  every  developed  character  is  represented 
in  the  germ  by  a  distinct  determiner,  or  inheritance  unit,  just  as 
it  is  not  necessary  to  suppose  that  every  chemical  compound  con- 
tains a  peculiar  chemical  element;  but  it  is  necessary  to  suppose 
that  each  hereditary  character  is  caused  by  some  particular  com- 
bination of  inheritance  units  and  that  each  compound  is  produced 
by  some  particular  combination  of  chemical  elements.  An  enor- 
mous number  of  chemical  compounds  exists  as  the  result  of  var- 
ious combinations  of  some  eighty  different  elements,  and  an  almost 
endless  number  of  words  and  combinations  of  words — indeed 


IO2  Heredity  and  Environment 

whole  literatures— may  be  made  with  the  twenty-six  letters  of  the 
alphabet.  It  is  quite  probable  that  the  kinds  of  inheritance  units 
are  few  in  number  as  compared  with  the  multitudes  of  adult  char- 
acters, and  that  different  combinations  of  the  units  give  rise  to 
different  adult  characters ;  but  it  is  certain  that  inherited  dif- 
ferences in  adult  organization  must  have  had  some  differential 
cause  or  factor  in  germinal  organization. 

Mendel  did  not  speculate  about  the  nature  of  hereditary  units 
though  he  evidently  conceived  that  there  was  something  in  the 
germ  which  corresponded  to  each  character  of  the  plant.  Weis- 
mann  postulated  a  determinant  in  the  germ  for  every  character 
which  is  independently  heritable,  and  many  recent  students  of 
heredity  hold  a  similar  view.  But  it  is  evident  that  there  is  not  an 
exact  one  to  one  correspondence  of  inheritance  units  and  adult 
characters.  Many  different  characters  may  be  determined  by  a 
single  unit  or  factor;  for  example,  all  the  numerous  secondary 
sexual  characters  which  distinguish  males  from  females  may  be 
determined  by  the  original  factor  which  determines  whether  the 
germ  cells  shall  be  ova  or  spermatozoa. 

Multiple  Factors. — On  the  other  hand  two  or  more  factors  may 
be  concerned  in  the  production  of  a  single  character.  In  many 
cases  among  both  plants  and  animals  the  development  of  color 
appears  to  depend  upon  the  presence  in  the  germ  cells  and  the 
cooperation  in  development  of  at  least  two  factors,  viz.  (i)  a  pig- 
ment factor  for  each  particular  color,  and  (2)  a  color  developer. 
When  both  of  these  factors  are  present  color  develops,  when 
either  one  is  absent  no  color  appears. 

Such  cases  have  been  described  for  mice,  guinea-pigs,  and  rab- 
bits as  well  as  for  several  species  of  plants.  Bateson  and  Pun- 
nett  found  two  varieties  of  white  sweet  peas  which  were  appar- 
ently alike  in  every  respect  except  the  shape  of  their  pollen  grains, 
one  of  them  having  long  and  the  other  round  pollen.  But  when 
these  were  crossed  a  remarkable  thing  occurred  for  the  progeny 
"instead  of  being  white  were  purple  like  the  wild  Sicilian  plant 


Phenomena  of  Inheritance  103 

from  which  our  cultivated  sweet  peas  are  descended."  This  is 
apparently  a  typical  case  of  reversion  and  its  cause  was  found  in 
the  fact  that  at  least  two  factors  are  necessary  in  this  case  for 
the  production  of  color,  a  pigment  factor  R  and  a  color  devel- 
oper C.  One  of  these  was  lacking  in  each  of  the  white  parents, 
their  gametic  formulae  being  Cr  and  cR  respectively,  but  when 
these  two  factors  came  together  in  the  offspring  a  purple-flowered 
type  was  produced  with  the  zygotic  formula  CcRr.  These  Fl 
plants  produced  colored  and  white  F2  plants  in  the  proportion  of  9 
colored  to  7  white  and  the  colored  forms  were  of  six  different 
kinds  (Fig.  33).  For  the  production  of  these  six  colored  forms 
five  different  factors  must  be  present  in  the  gametes,  according 
to  Punnett,  viz.:  (i)  a  color  base  R,  (2)  a  color  developer  C, 
(3)  a  purple  factor  P,  (4)  a  light  wing  factor  L,  (5)  a  factor  for 
intense  color  /.  When  all  of  these  factors  are  present  the  result 
is  the  purple  wild  form  with  blue  wings,  while  the  omission  of 
one  or  more  of  these  factors  leads  to  the  production  of  six 
forms  of  colored  and  various  types  of  white-flowered  plants  of 
the  F2  generation. 

Castle  found  that  eight  different  factors  may  be  involved  in 
producing  the  coat  colors  of  rabbits :  these  are : 

C  a  common  color  factor  necessary  to  produce  any  color. 

B  a  factor  acting  on  C  to  produce  black. 

Br  a  factor  acting  on  C  to  produce  brown. 

Y  a  factor  acting  on  C  to  produce  yellow. 

I  a  factor  which  determines  intensity  of  color. 

U  a  factor  which  determines  uniformity  of  color. 

A  a  factor  for  agouti,  or  wild  gray  pattern,  in  which  the  tip  of 
every  hair  is  black,  its  middle  yellow,  its  basal  part  gray. 

E  a  factor  for  the  extension  of  black  or  brown  but  not  of 
yellow. 

Plate  found  that  all  of  these  factors  except  the  last,  E,  are  also 
involved  in  the  production  of  the  coat  colors  of  mice.  Baur  has 
recognized  more  than  twenty  different  factors  for  the  color  and 
form  of  flowers  in  the  snapdragon,  Antirrhinum. 


1O4 


Heredity  and  Environment 


I         1      WHITE 

ll^-Vj      VERY  PALt  PIMPLE 

$M3      PINK 

PM.E  PURPLt 

FED 

PUKPLB 

Butt 

fiEB    DEEP  PURPLE 

FIG.  33.  RESULTS  OF  CROSSING  Two  DIFFERENT  RACES  (A  AND  5)  OF 
WHITE  SWEET  PEAS;  all  the  F^  hybrids  (C)  are  purple  with  blue  wings 
like  the  wild  ancestral  stock;  in  F2  six  colored  varieties  are  formed  rang- 
ing from  purple  with  blue  wings  (D)  to  tinged  white  (/)  and  several  kinds 
(genotypes)  of  white  varieties  (K).  (After  Punnett). 


Phenomena  of  Inheritance  105 

Modifying  Factors. — Morgan  and  Bridges  have  found  that  the 
effects  of  many  factors  may  be  modified  by  other  factors.  Thus 
the  eye  color  of  Drosophila  known  as  "eosin"  may  be  modified  by 
six  or  seven  different  factors,  occupying  different  loci  in  the 
chromosomes,  one  of  which  intensifies  "eosin"  while  the  others 
dilute  it.  These  modifying  factors  are  undoubtedly  like  other 
Mendelian  factors  in  their  behavior  and  they  show  that  an  adult 
character  may  be  the  result  of  several  different  inheritance  fac- 
tors. Indeed  Morgan  says  "that  an  overstatement  that  each  fac- 
tor may  affect  the  entire  body  is  less  likely  to  do  harm  than  to 
state  that  each  factor  affects  only  a  particular  character."  And 
again  he  says,  "It  cannot  too  insistently  be  urged  that  when  we 
say  a  character  is  the  product  of  a  particular  factor  we  mean  no 
more  than  that  it  is  the  most  conspicuous  effect  of  the  factor" 
(Morgan,  1916,  p.  117). 

Lethal  Factors. — Morgan  and  his  associates  have  also  demon- 
strated the  existence  of  a  considerable  number  of  lethal  factors 
in  Drosophila  that  cause  the  early  death  of  those  gametes  or 
zygotes  in  which  this  factor  is  not  balanced  by  a  normal  one. 
This  phenomenon  greatly  modifies  expected  Mendelian  ratios 
for  only  heterozygotes  survive,  and  all  individuals  that  are  homo- 
zygous  for  a  lethal  factor  usually  die  so  early  that  they  are  never 
seen.  Nevertheless  their  existence  can  be  determined  by  indirect 
methods  that  will  be  mentioned  in  the  next  chapter  under  "link- 
age." Such  lethal  factors  greatly  complicate  the  study  of  genetic? 
but  they  do  not  destroy  its  fundamental  principles. 

What  are  factors? — Inheritance  factors  are  probably  complex 
chemical  substances  which  preserve  their  individuality  in  various 
combinations,  just  as  groups  of  atoms  or  radicals  do  in  chemical 
reactions ;  they  may  be  dropped  out  or  added,  substituted  or  trans- 
posed, just  as  chemical  radicals  may  be  in  chemical  compounds. 
To  this  extent  they  maintain  continuity  and  independence,  but  they 
are  not  absolutely  independent  for  they  react  upon  one  another 
as  well  as  to  environmental  changes,  so  that  the  characters  of 


io6  Heredity  and  Environment 

the  developed  organism  are  the  resultants  of  all  these  reactions 
and  interactions. 

Some  progress  has  been  made,  in  identifying  certain  structures 
of  the  germ  cells  with  certain  hereditary  units,  but  quite  irrespec- 
tive of  what  these  units  may  be  and  where  they  may  be  located  it 
is  possible,  by  means  of  the  Mendelian  theory  of  segregation  of 
units  in  the  germ  cells  and  of  chance  combinations  of  these  in  fer- 
tilization to  predict  the  number  of  genotypes  and  phenotypes 
which  may  be  expected  as  the  result  of  a  given  cross. 

2.  Modifications  of  the  Principle  of  Dominance.  Incomplete 
Dominance. — A  large  number  of  animal  and  plant  hybrids  show 
one  contrasting  character  completely  dominant  over  the  other  one 
as  Mendel  observed  in  the  case  of  his  peas.  But  in  a  considerable 
number  of  cases  this  dominance  is  incomplete  or  imperfect.  When 
white-flowered  strains  of  "four  o'clocks"  are  crossed  with  red- 
flowered  ones  the  Fx  plants  bear  neither  white  nor  red  flowers 
but  pink  ones,  and  the  F2  plants  are  white-flowered,  red-flowered 
or  pink-flowered.  The  whites  and  reds  are  always  homozygous, 
the  pinks  heterozygous;  pure  white  and  pure  red  are  produced 
only  when  their  factors  are  duplex  (WW},  (RR)  ;  when  they  are 
simplex  (WR)  pink  is  produced.  In  this  case  red  is  not  com- 
pletely dominant  over  white,  but  the  hybrid  is  more  or  less  inter- 
mediate between  the  two  parents  (Fig.  28). 

It  has  long  been  known  that  the  race  of  fowls  called  Blue  An- 
dalusian  does  not  breed  true,  but  in  each  generation  produces  a 
certain  number  of  blacks  and  whites  as  well  as  blues.  Bateson 
found  that  the  blues  are  really  hybrids  between  blacks  and  whites 
in  which  neither  of  the  latter  is  completely  dominant.  Black  and 
white  appear  only  when  they  are  pure  (homozygous),  blue  only 
when  both  black  and  white  are  present (  heterozygous). 

Again  a  cross  of  red  and  white  cattle  produces  roan  offspring, 
but  the  latter  when  interbred  give  rise  to  reds,  roans  and  whites 
in  the  proportion  of  1:2:1,  showing  that  the  roans  are  heterozy- 
gotes  in  which  red  is  not  completely  dominant  over  white,  while 


Phenomena  of  Inheritance  107 

the  reds  and  whites  are  homozygotes  and  consequently  breed  true. 

'Lang  found  that  when  snails  with  uniformly  colored  shells 
were  crossed  with  snails  having  bands  of  color  on  the  shells  the 
hybrids  were  faintly  banded,  thus  being  more  or  less  interme- 
diate between  the  two  parents ;  but  when  these  hybrids  were  inter- 
bred they  produced  banded,  faintly  banded  and  uniformly  col- 
ored snails  in  the  ratio  of  1:2:1,  thus  proving  that  Mendelian 
segregation  takes  place  in  the  F2  generation,  and  that  dominance 
is  incomplete  in  the  heterozygotes.  Many  other  similar  cases  of 
incomplete  dominance  are  known. 

Sometimes  dominance  is  incomplete  in  early  stages  of  develop- 
ment but  becomes  complete  in  adult  stages.  Davenport  found  that 
when  white  and  black  fowls  are  crossed  the  chicks,  especially  the 
females,  are  speckled  white  and  black,  but  in  the  adult  fowl  domi- 
nance is  complete  and  the  plumage  is  white.  Similar  conditions  of 
delayed  dominance  are  well  known  in  the  color  of  hair  and  eyes 
of  children,  though  dominance  may  become  complete  when  they 
have  reached  adult  life. 

Reversible  Dominance. — In  a  few  instances  a  character  may  be 
dominant  at  one  time  and  recessive  at  another.  Thus  Davenport 
found  that  an  extra  toe  in  fowls  is  dominant  under  certain  cir- 
cumstances and  recessive  under  others.  Tennent  found  that  char- 
acters which  are  usually  dominant  in  hybrid  echinoderms  may  be 
made  recessive  if  the  chemical  or  physical  nature  of  the  sea  water 
is  changed.  Such  cases  seem  to  show  that  dominance  may  depend 
sometimes  upon  environmental  conditions,  sometimes  upon  a 
particular  combination  of  hereditary  units. 

Dominance  Not  Fundamental. — In  all  cases  dominance  means 
merely  the  development  in  offspring  of  certain  characters  of  one 
parent,  while  contrasting  characters  of  the  other  parent  remain 
undeveloped.  The  appearance  of  any  developed  character  in  an 
organism  depends  upon  many  complicated  reactions  of  germinal 
units  to  one  another  and  to  the  environment.  Under  certain  con- 
ditions of  the  germ  or  of  the  environment  some  characters  may 


io8  Heredity  and  Environment 

develop  in  hybrids  to  the  exclusion  of  their  opposites  whereas 
under  other  conditions  these  results  may  be  reversed  or  the  char- 
acters may  be  intermediate.  The  principle  of  dominance  is  not  a 
fundamental  part  of  Mendelian  inheritance.  Even  when  the 
characters  of  hybrids  are  intermediate  between  those  of  their 
parents,  if  the  parental  types  reappear  in  the  F2  generation  we 
may  be  certain  that  we  are  dealing  with  cases  of  Mendelian 
inheritance. 

3.  The  Principle  of  Segregation. — The  individuality  of  inheri- 
tance units,  and  their  segregation  or  separation  in  the  sex  cells 
and  recombination  in  the  zygote  are  fundamental  principles  of 
the  Mendelian  doctrine.  Indeed  the  evidence  for  the  individual- 
ity and  continuity  of  inheritance  units  is  based  entirely  upon  such 
segregation  and  recombination,  so  that  the  entire  Mendelian  theory 
may  be  said  to  rest  upon  the  principle  of  segregation.  If  there  are 
cases  in  which  such  segregation  does  not  take  place  they  belong 
to  other  forms  of  inheritance  than  the  Mendelian ;  if  segregation 
occurs  in  every  instance  there  is  no  other  type  of  inheritance  than 
that  discovered  by  Mendel.  Are  there  cases  which  do  not  segre- 
gate according  to  Mendelian  expectation  ? 

When  the  Mendelian  theory  was  new  it  was  generally  supposed 
that  there  were  forms  of  inheritance  which  differed  materially 
from  the  Mendelian  type ;  indeed  it  was  supposed  that  the  latter 
was  one  of  the  less  common  forms  of  heredity  and  that  blending 
of  parental  traits  and  not  segregation  was  the  rule.  All  cases  in 
which  the  characters  of  the  parents  appeared  to  blend  in  the 
offspring  or  in  which  there  was  not  a  clear  segregation  of  the  par- 
ental types  in  the  F2  generation  or  in  which  the  ratio  of  dominants 
to  recessives  differed  from  the  well  kmfwn  3  to  I  ratio  were  sup- 
posed to  be  non-Mendelian. 

Unusual  Ratios. — However  further  work  has  shown  that  most 
of  these  cases  are  really  Mendelian.  Sometimes  offspring  are  in- 
termediate between  their  parents  owing  to  incompleteness  of  domi- 
nance, rather  than  to  incompleteness  of  segregation ;  in  such  cases 


Phenomena  of  Inheritance  109 

the  parental  types  reappear  in  the  F2  generation  as  in  the  cross 
between  red  and  white  "four  o'clocks."  Sometimes  departures 
from  the  3  to  i  ratio  are  caused  by  the  fact  that  two  or  more  fac- 
tors of  the  same  sort  are  involved  in  the  production  of  a  single 
character.  Nilsson-Ehle  found  that  when  oats  with  black  glumes 
were  crossed  with  varieties  having  white  glumes  the  ratio  of  3 
white  to  i  black  was  usually  found  in  the  second  generation ;  but 
one  variety  of  black  oats  when  crossed  with  white  gave  in  the 
second  generation  approximately  15  blacks  to  i  white  which  is 
the  dihybrid  ratio.  From  this  and  other  evidence  he  concludes 
that  in  this  variety  of  oats  two  hereditarily  separable  factors  are 
involved  in  the  production  of  black.  In  crosses  between  red- 
grained  and  white-grained  wheat  he  usually  got  in  the  second  gen- 
eration the  monohybrid  ratio  of  3  red  to  i  white,  but  three  strains 
gave  the  dihybrid  ratio  of  15  to  i  and  two  gave  the  trihybrid  ratio 
of  63  to  i  and  in  subsequent  generations  each  of  these  strains 
continued  to  give  the  same  ratios.  Consequently  he  concludes 
that  while  the  red  color  of  wheat  grains  is  usually  due  to  one 
factor  for  red,  it  may  in  some  cases  be  due  to  two  or  even  to 
three  factors;  notable  departures  from  expected  ratios  may  thus 
be  explained.  Other  departures  from  regular  Mendelian  ratios 
are  caused  by  the  early  death  of  certain  gametes  or  zygotes  due 
to  lethal  factors,  as  explained  on  page  105. 

Blending  of  Color  in  Mulatto. — Perhaps  the  most  serious  objec- 
tions which  can  be  presented  against  the  universality  of  the  Men- 
delian doctrine  are  found  in  phenomena  of  "blending"  inheritance. 
In  some  instances  contrasting  characters  of  parents  appear  to  blend 
in  offspring  and  even  in  the  F2  and  in  subsequent  generations  the 
descendants  remain  more  or  less  intermediate  between  the  parents. 
One  of  the  best  known  illustrations  of  this  is  found  in  the  skin  col- 
or of  the  mulatto  which  is  intermediate  between  the  white  parent 
and  the  black  one,  and  even  in  the  F2  and  in  subsequent  generations 
mulattoes  do  not  usually  produce  pure  white  or  pure  black  chil- 
dren, though  the  children  of  mulattoes  show  considerable  variation 


no  Heredity  and  Environment 

in  color.  Hence  there  seems  to  be  a  failure  of  the  Mendelian 
principle  of  segregation. 

But  white  skin  is  not  really  white  nor  is  black  skin  ever  perfect- 
ly black.  Davenport  has  shown  that  there  is  a  mixture  of  black, 
yellow  and  red  pigments  in  both  white  and  black  skins,  though  the 
amount  of  each  of  these  pigments  varies  greatly  in  negroes  and 
whites.  The  relative  amounts  of  these  pigments  in  any  given  case 
may  be  determined  by  means  of  a  rotating  color  disk.  A  white 
person  may  have  a  skin  color  composed  of  black  (b)  8  per  cent, 
yellow  (y)  9  per  cent,  red  (r)  50  per  cent,  and  absence  of  pigment 
or  white  (w)  33  per  cent.  On  the  other  hand  a  very  black 
negro  may  have  b  68  per  cent,  y  2  per  cent,  r  26  per  cent,  w  4 
per  cent.  The  nine  children  of  two  mulattoes,  the  father  having 
13  per  cent  of  black  and  the  mother  45  per  cent,  ranged  all  the 
way  from  46  per  cent  to  6  per  cent  of  black,  the  latter  so  far  as 
skin  color  is  concerned  being  virtually  white.  On  the  other  hand 
where  both  parents  have  about  the  same  degree  of  pigmentation 
the  children  are  more  nearly  uniform  in  color;  thus  seven  chil- 
dren of  two  mulattoes,  the  father  having  36  per  cent  and  the 
mother  30  per  cent  of  black,  ranged  only  from  27  per  cent  to  39 
per  cent  of  black.* 

Such  variations  in  color  in  the  F2  and  in  subsequent  genera- 
tions are  exactly  what  one  would  expect  in  a  Mendelian  character 
in  which  more  than  one  factor  is  involved,  as  for  example  in  the 
case  of  the  color  of  the  sweet  peas  shown  in  Fig.  33.  Davenport, 
who  has  made  an  extensive  study  of  this  case,  concludes  that 
"there  are  two  double  factors  (A A,  BB)  for  black  pigmentation 
in  the  full  blooded  negro  of  the  west  coast  of  Africa,  and  these 
are  separably  inheritable."  These  factor^  are  lacking  in  white 
persons  (this  being  indicated  by  the  formula  aa,  bb).  Since  the 
germ  cells  carry  only  single  factors  and  not  double  ones  the  cross 
between  negro  and  white  would  have  only  one  set  of  these  fac- 

*  In  another  family  shown  in  Fig.  35  the  father  has  18  per  cent  black 
pigment,  the  mother  38  per  cent  and  the  children  range  from  17  per  cent 
to  54  per  cent. 


Phenomena  of  Inheritance 


ill 


AB         Ab 


aB 


a  b 


V 
AB 

Ab 
aB 
a  b 

AB 
AB 

Ab 
AB 

aB 
AB 

ab 
AB 

AB 
Ab 

Ab 
Ab 

aB 
Ab 

ab 
Ab 

AB 
aB 

Ab 
aB 

aB 
'aB 

ab 
aB 

AB 

a  b 

Ab 
ab 

aB 
ab 

a  b 
a  b 

FIG.  34.  CHECKERBOARD  DIAGRAM  SHOWING  RESULTS  OF  CROSSING  Two 
MULATTOES,  each  having  color  factors  ABab.  Types  of  male  gerih  cells 
are  above  the  square,  of  female  cells  on  the  left  and  the  possible  combina- 
tions of  these  are  shown  in  the  16  small  squares.  Homozygotes  are  found 
only  along  the  diagonal.  The  color  of  the  children  varies  all  the 
way  from  black  (upper  left  corner)  to  white  (lower  right  corner). 


FIG.  35.  MULLATTO  HUSBAND  AND  WIFE  AND  THEIR  SEVEN  CHILDREN 
ranging  in  color  from  the  one  on  left  who  "passes  for  white"  to  the 
youngest  who  is  typically  black.  (From  Davenport.) 


112  Heredity  and  Environment 

tors  for  black  color,  as  shown  by  the  formula  AB  x  ab  =  ABab ; 
hence  the  color  of  the  Ft  generation  is  intermediate  between  that 
of  the  two  parents.  In  the  F2  generation  there  should  be  a  variety 
of  colors  ranging  all  the  way  from  white  to  black  (Fig  34),  though 
pure  white  (ab  ab)  or  pure  black  AB  AB)  would  be  expected  in 
only  i  out  of  16  of  the  offspring.  As  a  matter  of  fact  it  is  known 
that  the  children  of  mulattoes  vary  considerably  in  color,  and  in 
some  cases  a  child  may  be  darker  or  lighter  than  either  parent, 
which  would  indicate  that  segregation  does  actually  occur.  It  is 
very  probable  that  this  classical  case  of  "blending"  inheritance  is 
really  Mendelian  inheritance  in  which  two  or  more  factors  for 
skin  color  are  involved. 

Blending  of  Size. — Similar  "blending"  inheritance  is  found  in 
certain  other  cases  where  the  parents  differ  in  form  or  size.  Thus 
Castle  found  that  when  long-eared  rabbits  were  crossed  with 
short-eared  ones  the  offspring  have  ears  of  intermediate  length, 
and  in  all  subsequent  generations  the  ear  length  remained  inter- 
mediate between  that  of  the  parents.  He  found  the  same  thing 
true  of  length  and  breadth  of  the  skull  (Fig.  36)  and  of  the 
size  of  other  portions  of  the  skeleton,  and  he  concluded  that 
such  quantitative  characters  are  not  inherited  in  Mendelian 
fashion. 

More  recently  MacDowell,  working  on  the  inheritance  of  size 
in  rabbits,  concludes  that  this  character  as  well  as  other  quan- 
titative differences  between  parents  which  appear  to  blend  in  the 
offspring,  such  as  Castle's  case  of  ear  length  in  rabbits,  is  not  due 
to  a  single  factor,  as  in  the  case  of  Mendel's  tall  and  dwarf  peas, 
but  to  several  factors.  Consequently  in  the  formation  of  the  germ 
cells  there  is  not  a  clean  segregation  of  all  the  factors  for  tallness 
or  large  size  or  long  ears  in  half  the  germ  cells  and  their  total 
absence  in  the  other  half  of  those  cells,  but  some  of  these  factors 
go  into  certain  cells  and  others  into  others,  as  in  the  case  of  dihy- 
brids,  trihybrids  or  polyhybrids.  As  a  result  offspring  appear 
more  or  less  intermediate  in  size  between  their  parents. 

Thus  it  is  possible  to  explain  even  "blending"  inheritance  as 


Phenomena  of  Inheritance 

due  not  to  the  real  fusion  or  blending  of  inheritance  factors  but 
-to  varying  combinations  of  numerous  or  multiple  factors,  accord- 
ing to  the  Mendelian  rules.  The  Mendelian  principle  of  segrega- 
tion has  been  found  to  be  of  such  general  occurrence  that  there 
is  a  strong  probability  that  it  is  universal,'  and  that  all  cases  of 
"blending"  inheritance  are  due  to  incomplete  dominance  or  to 
multiple  factors. 

Maternal  Inheritance. — Another  case  which  seems  at  first 
sight  to  be  non-Mendelian  is  what  may  be  called  "maternal  in- 
heritance" since  certain  characters  are  invariably  derived  from  the 
mother  and  not  from  the  father.  Among  these  are  the  polarity, 
symmetry  and  pattern  of  the  egg  and  of  the  adult  animal  which 
is  derived  from  it  (see  p.  196).  These  characters  are  of  such  a 
general  sort  that  they  may  not  be  recognized  as  phenomena  of 
inheritance  at  all,  and  yet  they  form  the  background  and  frame- 
work for  all  the  other  characters.  They  do  not  come  equally 


FIG.  36.     INHERITANCE  OF  SIZE  IN  RABBITS.    The  skulls  of  two  parents 
are  shown  in  I  and  3,  of  their  intermediate  offspring  in  2.     (From  Castle.) 


Heredity  and  Environment 


from  the  egg  and  sperm,  and  they  do  not  undergo  segregation  in 
the  formation  of  the  gametes,  but  are  apparently  derived  from  the 
egg  cytoplasm.  Among  characters  of  this  sort  are  the  normal 
and  inverse  symmetry  of  snails,  and  of  many  other  animals, 
including  man,  which  are  referred  to  on  pages  197-205.  Such 
characters  are  undoubtedly  inherited,  though  they  differ  from 
other  characters  not  only  in  the  fact  that  they  are  transmitted 
through  the  egg  only,  but  also  because  they  are  of  the  same  kind 
in  the  egg  and  in  the  developed  organism ;  they  are  in  a  measure 
preformed  in  the  egg;  they  are  differentiated  characters  carried 
over  from  a  previous  generation  rather  than  inheritance  factors. 
These  egg  characters  probably  appeared  in  the  course  of  oogenesis 
under  the  influence  of  paternal  as  well  as  of  maternal  factors ; 
if  so  this  is  a  case  of  Mendelian  inheritance  in  the  previous 
generation  or  what  may  be  called  "Pre-inheritance."  Similar 
phenomena  have  been  described  by  McCracken  and  by  Toyama 
in  silk-worms  where  several  egg  characters  seem  to  be  non- 
Mendelian,  but  Toyama  has  shown  that  they  are  in  reality  Men- 


FIG.  37.  X-RAY  PICTURE  OF  RIGHT  AND  LEFT  HANDS  EACH  WITH  Six 
FINGERS  (polydactyly)  caused  by  splitting  of  the  little  fingers  at  an  early 
stage.  (From  Journal  of  Heredity.) 


Phenomena  of  Inheritance  115 

delian  in  the  previous  generation,  this  also  being  a  case  of  pre- 
inheritance. 

It  has  been  found  by  Correns,  Baur,  and  Shull  that  the  leaf 
colors  of  certain  plants  are  not  inherited  in  Mendelian  fash- 
ion, but  the  chromoplasts,  which  produce  the  chromatophores 
(chloroplasts),  are  transmitted  from  one  generation  to  the  next 
in  the  cytoplasm  of  the  egg  cell  and  only  rarely  through  the 
male  sex  cell.  If  chromoplasts  are  integral  parts  of  a  plant 
and  undergo  differentiation  or  development  this  may  be  a  case  of 
pre-inheritance ;  if  they  are  symbiotic  organisms  it  is  an  instance 
of  the  inclusion  of  foreign  bodies  in  the  cytoplasm  and  not  in- 
heritance at  all. 

Other  forms  of  transmission  are  known  in  which  substances 
are  carried  over  from  one  generation  to  the  next  through  the  egg, 
but  they  are  probably  not  cases  of  true  inheritance.  Among  these 
are  the  occasional  transmission  of  immunity  through  the  mother 
but  never  through  the  father,  the  carrying  over  of  particular 
chemical  substances  such  as  fat  dyes  through  the  egg  but  not 
through  the  sperm,  and  the  transport  of  symbiotic  or  parasitic 
organisms,  such  as  algae,  bacteria,  etc.,  through  the  female  sex 
cell  but  not  through  the  male  cell.  These  substances  or  micro- 


itli 

iw 
% 


B  A 

FIG.   38.    X-RAY   PICTURE,   A   OF   A   NORMAL,   B   OF   A    SHORT-FINGERED 
(brachydactyl)   hand.     (From  Bateson.) 


u6  Heredity  and  Environment 

organisms  are  to  be  regarded  as  inclusions  in  the  egg  rather  than 
as  any  permanent  part  of  the  germinal  organization;  consequently 
they  are  not  inherited  in  the  strict  sense  of  that  term. 

III.    MENDELIAN  INHERITANCE  IN  MAN 

The  study  of  inheritance  in  man  must  always  be  less  satisfac- 
tory and  the  results  less  secure  than  in  the  case  of  lower  animals 
and  for  the  following  reasons :  In  the  first  place  there  are  no 
"pure  lines"  but  the  most  complicated  intermixture  of  different 
lines.  In  the  second  place  experiments  are  out  of  the  question 
and  one  must  rely  upon  observation  and  statistics.  In  the  third 
place  man  is  a  slow  breeding  animal;  there  have  been  less  than 
sixty  generations  of  men  since  the  beginning  of  the  Christian  era, 
whereas  Jennings  gets  as  many  generations  of  Paramecium  with- 
in two  months  and  Morgan  almost  as  many  generations  of  Dro- 
sophila  within  two  years.  Finally  the  number  of  offspring  are 
so  few  in  human  families  that  it  is  impossible  to  determine  what  all 
the  hereditary  possibilities  of  a  family  may  be.  Bearing  in  mind 
these  serious  handicaps  to  an  exact  study  of  inheritance  it  is  not 
surprising  that  the  method  of  inheritance  of  many  human  char- 
acters is  still  uncertain. 

Davenport  and  Plate  have  catalogued  more  than  sixty  human 
traits  which  seem  to  be  inherited  in  Mendelian  fashion.  About 
fifty  of  these  represent  pathological  or  teratological  conditions 
while  only  a  relatively  small  number  are  normal  characters.  This 
does  not  signify  that  the  method  of  inheritance  differs  in  the 
the  case  of  normal  and  abnormal  characters,  but  rather  that  ab- 
normal characters  are  more  striking,  more/  easily  followed  from 
generation  to  generation,  and  consequently  statistics  are  more 
complete  with  regard  to  them  than  in  the  case  of  normal  char- 
acters. In  many  cases  statistics  are  not  sufficiently  complete  to 
determine  with  certainty  whether  the  character  in  question  is 
dominant  or  recessive,  and  it  must  be  understood  that  in  some 
instances  the  classification  in  this  respect  is  tentative.  A  par- 
tial list  of  these  characters  is  given  herewith: 


Phenomena  of  Inheritance 


117 


nS 


Heredity  and  Environment 


MENDELIAN  INHERITANCE  IN  MAN 


NORMAL    CHARACTERS 


Recessive 


Dominant 
Hair: 

Curly  Straight 

Dark  Light  to  red 

Eye  Color; 

Brown  Blue 

Skin  Color: 

Dark  Light 

Normal  Pigmentation  Albinism 

Countenance: 

Hapsburg    Type    (Thick    lower  Normal 

lip  and  prominent  chin) 
Temperament: 

Nervous  Phlegmatic 

Intellectual  Capacity: 

Average  Very  great 

Average  Very  small 

TERATOLOGICAL    AND    PATHOLOGICAL    CHARACTERS 

General  Size; 
Achondroplasy     (Dwarfs     with  Normal 

short    stout    limbs    but    with 

bodies  and  heads   of   normal 

size) 

Normal  size  True  Dwarfs   (With  all  parts  of 

the  body  reduced  in  proportion) 
Hands  and  Feet: 

Brachydactyly      (Short     fingers  Normal     (Fig.  38) 

and  toes) 
Syndactyly      (Webbed      fingers  Normal 

and  toes) 
Polydactyly        (Supernumerary  Normal     t^Tg.  37) 

digits) 
Skin: 
Keratosis    (Thickening  of   Epi-  Normal 

dermis) 
Epidermolysis    (Excessive    for-  Normal 

mation  of  blisters) 
Hypotrichosis  (Hairlessness  as-  Normal 

sociated  with  lack  of  teeth) 


Phenomena  of  Inheritance 


119 


MENDELIAN  INHERITANCE  IN  MAN  (Continued) 


TERATOLOG1CAL    AND 

Dominant 
Kidneys; 

Diabetes   insipidus 
Diabetes   mellitus 
Normal 

Nervous  System: 
Normal  Condition 


Nervous  System: 
Normal 

Normal 


Normal 

Normal 
Normal 

Huntington's  Chorea 
Muscular  Atrophy 
Eyes: 

Hereditary  Cataract 
Pigmentary      Degeneration      of 

Retina 
Glaucoma      (Internal     pressure 

and  swelling  of  eyeball) 
Coloboma      (Open      suture      in 

iris) 

Displaced  Lens 
Ears.- 
•Normal 
Normal 


PATHOLOGICAL    CHARACTERS 

Recessive 

Normal 
Normal 

Alkaptonuria    (Urine    dark    after 
oxidation) 

General  Neuropathy,  e.g. 

Hereditary  Epilepsy 

Hereditary  Feeble-mindedness 

Hereditary  Insanity 

Hereditary  Alcoholism 

Hereditary  Criminality 

Hereditary  Hysteria 

Multiple  Sclerosis  (Diffuse  de- 
generation of  nerve  tissue) 

Friedrich's  Disease  (Degenera- 
tion of  upper  part  of  spinal 
cord) 

Meniere's  Disease  (Dizziness 
and  roaring  in  ears) 

Chorea  (St.  Vitus  Dance) 

Thomsen's  Disease  (Lack  of 
muscular  tone) 

Normal 

Normal 
» 

Normal 

Normal 

Normal 
Normal 
Normal 

Deaf-mutism 

Otosclerosis  (Rigidity  of  tym- 
panum, etc.,  with  .hardness  of 
hearing) 


I2O  Heredity  and  Environment 

SEX-LINKED    CHARACTERS* 

Recessive  characters,  appearing  in  male  when  simplex,  in  female  when 
duplex. 

Normal  Gower's  Muscular  Atrophy 

Normal  Haemophilia  (Slow  clotting  of 

blood) 

Normal  Color  Blindness  (Daltonism;  in- 

ability to  distinguish  red  from 
green) 

Normal  Night  Blindness  (Inability  to  see 

by  faint  light) 

Normal  Neuritis  Optica  (Progressive 

atrophy  of  optic  nerve) 

SUMMARY 

The  principles  of  heredity  established  by  Mendel  are  almost  as 
important  for  biology  as  the  atomic  theory  of  Dalton  is  for  chem- 
istry. By  means  of  these  principles  particular  dissociations  and 
recombinations  of  characters  can  be  made  with  almost  the  same 
certainty  as  particular  dissociations  and  recombinations  of  atoms 
can  be  made  in  chemical  reactions.  By  means  of  these  principles 
the  hereditary  constitution  of  organisms  can  be  analyzed  and  the 
real  resemblances  and  differences  of  various  organisms  deter- 
mined. By  means  of  these  principles  the  once  mysterious  and 
apparently  capricious  phenomena  of  prepotency,  atavism  and 
reversion  find  a  satisfactory  explanation. 

Before  the  establishment  of  Mendel's  principles,  heredity  was, 
as  Balzac  said,  "a  maze  in  which  science  loses  itself."  Much  still 
remains  to  be  discovered  about  inheritance,  but  the  principles  of 
Mendel  have  served  as  an  Ariadne  thread  to  guide  science 
through  this  maze  of  apparent  contradictions  and  exceptions  in 
which  it  was  formerly  lost. 

*  See  page  187. 


CHAPTER  III 

THE  CELLULAR  BASIS  OF  HEREDITY 
AND  DEVELOPMENT 


CHAPTER  III 


THE  CELLULAR  BASIS   OF  HEREDITY  AND 
DEVELOPMENT 

A.     INTRODUCTORY 

Heredity  is  to-day  the  central  problem  of  biology.  This  prob- 
lem may  be  approached  from  many  sides,  that  of  the  observer,  the 
statistician,  the  practical  breeder,  the  experimenter,  the  embry- 
ologist,  the  cytologist;  but  these  different  aspects  of  the  subject 
may  be  reduced  to  three  general  methods  of  study,  (i)  the  ob- 
servational and  statistical,  (2)  the  experimental,  (3)  the  cyto- 
logical  and  embryological.  We  have  dealt  with  the  first  and  sec- 
ond of  these  in  the  preceding  chapter  and  before  taking  up  the 
third  it  is  important  that  we  should  have  clear  definitions  of  the 
terms  employed  and  a  fairly  accurate  conception  of  the  processes 
involved. 

i.  Confused  Ideas  of  Heredity. — Heredity  originally  meant  the 
transmission  of  property  from  parents  to  children,  and  in  the 
field  of  biology  it  has  been  defined  erroneously  as  "the  transmis- 
sion of  qualities  or  characteristics,  mental  or  physical,  from  par- 
ents to  offspring."*  The  colloquial  meaning  of  the  word  has  led 
to  much  confusion  in  biology,  for  it  carries  with  it  the  idea  of 
the  transmission  from  one  generation  to  the  next  of  ownership 
in  property.  A  son  may  inherit  a  house  from  his  father  and  a 
farm  from  his  mother,  the  house  and  farm  remaining  the  same 
though  the  ownership  has  passed  from  parents  to  son.  And 
when  it  is  said  that  a  son  inherits  his  stature  from  his  father  and 
his  complexion  from  his  mother,  the  stature  and  complexion  are 
usually  thought  of  only  in  their  developed  condition,  while  the 
great  fact  of  development  is  temporarily  forgotten.  Of  course 

*  Century  Dictionary. 

123 


124  Heredity  and  Environment 

there  are  no  "qualities"  or  "characteristics"  which  are  "trans- 
mitted" as  such  from  one  generation  to  the  next.  Such  terms 
are  not  without  fault  when  used  merely  as  figures  of  speech,  but 
when  interpreted  literally,  as  they  frequently  are;  they  are  alto- 
gether misleading;  they  are  the  result  of  reasoning  about  names 
rather  than  facts,  of  getting  far  from  phenomena  and  philosophiz- 
ing about  them.  The  comparison  of  heredity  to  the  transmission 
of  property  from  parents  to  children  has  produced  confusion  in 
the  scientific  as  well  as  in  the  popular  mind.  It  is  only  necessary 
to  recall  the  most  elementary  facts  about  development  to  recog- 
nize that  in  a  literal  sense  developed  characteristics  of  parents  are 
never  transmitted  to  children. 

2.  The  Transmission  Hypothesis. — And  yet  the  idea  that  the 
characteristics  of  adult  persons  are  transmitted  from  one  genera- 
tion to  the  next  is  a  very  ancient  one  and  was  universally  held 
until  the  most  recent  times.  Before  the  details  of  development 
were  known  it  was  natural  to  suppose,  as  Hippocrates  did,  that 
white-flowered  plants  gave  rise  to  white-flowered  seeds  and  that 
blue-eyed  parents  produced  blue-eyed  germs,  without  attempt- 
ing to  define  what  was  meant  by  white-flowered  seeds  or  blue- 
eyed  germs.  And  even  after  the  facts  of  development  were  fairly 
well  known  it  was  generally  held  that  the  germ  cells  were  made 
by  the  adult  animal  or  plant  and  that  the  characteristics  of  the 
adult  were  in  some  way  carried  over  to  the  germ  cells;  but  the 
manner  in  which  this  supposed  transmission  took  place  remained 
undefined  until  Darwin  attempted  to  explain  it  by  his  "provisional 
hypothesis  of  pangenesis."  Darwin  assumed  that  minute  parti- 
cles or  "gemmules"  were  given  off  by/ every  cell  of  the  body,  at 
every  stage  of  development,  and  that  these  gemmules  then  col- 
lected in  the  germ  cells  which  thus  became  storehouses  of  little 
germs  from  all  parts  of  the  body.  Afterward,  in  the  develop- 
ment of  the  embryo,  the  gemmules,  or  little  germs,  developed 
into  cells  and  organs  similar  to  those  from  which  they  originally 
came. 


The  Cellular  Basis  125 

3.  Germinal  Continuity  and  Somatic  Discontinuity. — Many 
ingenious  hypotheses  have  been  devised  to  explain  things  which  are 
not  real,  and  this  is  one  of  them.  The  doctrine  that  adult  organ- 
isms manufacture  germ  cells  and  transmit  their  characters  to  them 
is  now  known  to  be  erroneous.  Neither  germ  cells  nor  any  other 
kind  of  cells  are  formed  by  the  body  as  a  whole,  but  every  cell  in 
the  body  comes  from  a  preceding  cell  by  a  process  of  division, 
and  germ  cells  are  formed,  not  by  contributions  from  all  parts  of 
the  body,  but  by  division  of  preceding  cells  which  are  derived 
ultimately  from  the  fertilized  egg  (Fig.  40).  The  hen  does  not 
produce  the  egg,  but  the  egg  produces  the  hen  and  also  other 
eggs.  Individual  traits  are  not  transmitted  from  the  hen  to  the 
egg,  but  they  develop  out  of  germinal  factors  which  are  carried 
along  from  cell  to  cell,  and  from  generation  to  generation. 

Germ  Cells  and  Body  Cells. — There  is  a  continuity  of  germinal 
substance,  and  usually  of  germinal  cells,  from  one  generation  to 
the  next.  In  some  animals  the  germ  cells  are  set  apart  at  a  very 
early  stage  of  development,  sometimes  in  the  early  cleavage  stages 
of  the  egg.  In  other  cases  the  germ  cells  are  first  recognizable  at 
later  stages,  but  in  practically  every  case  they  arise  from  germinal 
or  embryonic  cells  which  have  not  differentiated  into  somatic 
tissues.  In  general  then  germ  cells  do  not  come  from  differen- 
tiated body  cells,  but  only  from  undifferentiated  germinal  cells, 
and  if  in  a  few  doubtful  cases  differentiated  cells  may  reverse  the 
process  of  development  and  become  embryonic  cells  and  even 
germ  cells  it  does  not  destroy  this  general  principle  of  germinal 
continuity  and  somatic  discontinuity  of  successive  generations. 

Thus  the  problem  which  faces  the  student  of  heredity  and  de- 
velopment has  been  cut  in  two;  he  no  longer  inquires  how  the 
body  produces  the  germ  cells,  for  this  does  not  happen,  but  merely 
how  the  latter  produce  the  body  and  other  germ  cells.  The  germ 
is  the  undeveloped  organism  which  forms  the  bond  between  suc- 
cessive generations;  the  body  is  the  developed  organism  which 
arises  from  the  germ  under  the  influence  of  environmental  con- 


126 


Heredity  and  Environment 


ancles  <$  \\ 


FIG.  40.  DIAGRAM  SHOW- 
ING THE  "CELL  LINEAGE"  OF 
THE  BODY  CELLS  AND  GERM 
CELLS  IN  A  WORM  OR  MOL- 
LUSK.  The  lineage  of  the 
germ  cells  ("germ  track") 
is  shown  in  black,  of  ecto- 
derm in  white,  and  of  endo- 
derm  and  mesoderm  in 
shaded  circles.  The  whole 
course  of  spermatogenesis 
and  oogenesis  is  shown  in 
the  lower  right  of  the  figure 
beginning  with  the  primitive 
sex  cells  (Prim.  Sex  Cells) 
and  ending  with  the  gam- 
etes, the  genesis  of  the  sper- 
matozoa being  shown  on  the 
left  and  that  of  the  ova  on 
the  right. 


Ogte*ll+    I   , 

»«*J\  V 

Tid»  •  •  *  4 


Kb. 


Gametes 


ditions.  The  body  develops  and  dies/in  each  generation;  the 
germplasm  is  the  continuous  stream  of  living  substance  which 
connects  all  generations.  The  body  nourishes  and  protects  the 
germ;  it  is  the  carrier  of  the  germplasm,  the  mortal  trustee  of  an 
immortal  substance. 

4.  Germplasm  and  Somato plasm. — This  contrast  between  the 
germ  and  the  body,  between  the  undeveloped  and  the  developed 


The  Cellular  Basis 


127 


FIG.  41 

organism,  is  fundamental  in  all  modern  studies  of  heredity.  It  was 
especially  emphasized  by  Weismann .  in  his  germplasm  theory 
and  recently  it  has  been  made  prominent  by  Johannsen  under  the 
terms  "genotype"  and  "phenotype" ;  the  genotype  is  the  funda- 
mental hereditary  constitution  of  an  organism,  it  is  the  germinal 
type;  the  phenotype  is  the  developed  organism  with  all  of  its 
visible  characters,  it  is  the  somatic  type. 

But  important  as  this  distinction  is  between  germ  and  soma 
it  has  sometimes  been  overemphasized.  This  is  one  of  the  chief 
faults  of  Weismann's  theory.  The  germ  and  the  soma  are  generi- 
cally  alike,  but  specifically  different.  Both  germ  cells  and  somatic 
cells  have  come  from  the  same  oosperm,  but  have  differentiated  in 
different  ways ;  the  tissue  cells  have  lost  certain  things  which  the 
germ  cells  retain  and  have  developed  other  things  which  remain 
undeveloped  in  the  germ  cells.  But  the  germ  cells  do  not  remain 


128  Heredity  and  Environment 

undifferentiated ;  both  egg  and  sperm  are  differentiated,  the  for- 
mer for  receiving  the  sperm  and  for  the  nourishment  of  the  em- 
bryo, the  latter  for  locomotion  and  for  penetration  into  the  egg. 
But  while  the  differentiations  of  tissue  cells  are  usually  irreversi- 
ble, so  that  they  do  not  again  become  germinal  cells,  the  differen- 
tiations of  the  sex  cells  are  reversible,  so  that  these  cells,  after 
their  union,  again  become  germinal  cells.  The  ovum  loses  its 
power  to  form  yolk  and  during  the  early  development  it  gradually 
loses  all  the  yolk  which  it  had  stored  up ;  the  spermatozoon  loses 
its  highly  differentiated  tail  or  locomotor  apparatus  and  its  small 
compact  nucleus  absorbs  substance  from  the  cytoplasm  of  the 
egg  and  becomes  a  large  germinal  nucleus. 

Chromatin  is  Germplasm,  Cytoplasm  is  Somatoplasm. — In  many 
theories  of  heredity  it  is  assumed  that  there  is  a  specific  "inheri- 
tance material,"  distinct  from  the  general  protoplasm,  the  func- 
tion of  which  is  the  "transmission"  of  hereditary  properties  from 
generation  to  generation,  and  the  chief  characteristics  of  which 
are  independence  of  the  general  protoplasm,  continuity  from 
generation  to  generation  and  extreme  stability  in  organization. 
This  is  the  idioplasm  of  Nageli,  the  germplasm  of  Weismann. 
Such  a  substance  is  no  mere  fiction  or  logical  abstraction,  as 
many  writers  have  affirmed,  for  there  is  in  the  nucleus  of  every 
cell  a  substance  which  fulfills  all  of  these  conditions,  namely, 
the  chromatin.  It  is  relatively  independent  of  the  surrounding 
cytoplasm,  it  is  self-propagating  and  consequently  continuous 
from  cell  to  cell,  and  from  generation  to  generation  and  it  is 
relatively  stable  in  organization  so  that  it  is  but  little  influenced 
by  environmental  conditions.  There  are  many  important  rea- 
sons for  believing  that  the  chromatin  is  the  germplasm,  or  at 
least  that  it  contains  the  inheritance  units,  as  we  shall  see  later. 
It  is  present  not  only  in  germ  cells  but  in  every  cell  of  the  organ- 
ism, though  in  highly  differentiated  tissue  cells  it  may  undergo 
certain  secondary  modifications.  On  the  other  hand  the  cyto- 
plasm surrounding  the  nucleus,  undergoes  many  marked  differ- 


The  Cellular  Basis  129 

entiations  in  the  course  of  development  and  it  constitutes  in  the 
main  the  body  plasm  or  somatoplasm.  Germplasm  and  somato- 
plasm  are  not,  therefore,  vague  generalizations,  but  they  are  defin- 
ite cell  substances  which  may  be  seen  under  the  microscope. 

5.  The  Units  of  Living  Matter. — The  entire  cell,  nucleus  and 
cytoplasm,  is  the  smallest  unit  of  living  matter  which  is  capable 
of  independent  existence.  Neither  the  nucleus  nor  the  cytoplasm 
can  for  long  live  independently  of  each  other,  but  the  entire  cell 
can  perform  all  the  fundamental  vital  processes.  It  transforms 
food  into  its  own  living  material,  it  grows  and  divides,  it  is  capa- 
ble of  responding  to  many  kinds  of  stimuli.  But  while  the  parts  of 
a  cell  are  not  capable  of  independent  existence  they  may  be  dif- 
ferentiated to  perform  different  functions. 

Panmerism. — Not  only  is  the  cell  as  a  whole  capable  of  assimi- 
lation, growth  and  division,  but  every  visible  part  of  the  cell  has 
this  power.  The  nucleus  builds  foreign  substances  into  its  own 
substance,  and  after  it  has  grown  to  a. certain  size  it  divides  into 
two;  the  cytoplasm  does  the  same,  and  this  process  of  assimila- 
tion, growth  and  division  occurs  in  manv  parts  of  the  nucleus 
and  cytoplasm,  such  as  the  chromosomes,  chromomeres,  centro- 
sorries,  etc.  In  all  cases  cells  come  from  cells,  nuclei  from  nuclei, 
chromosomes  from  chromosomes,  centrosomes  from  centro- 
somes,  etc. 

Indeed,  the  manner  in  which  all  living  matter  grows  indicates 
that  every  minute  particle  of  protoplasm  has  this  power  of  taking 
in  food  substance  and  of  dividing  into  two  particles  when  it  has 
grown  to  maximum  size ;  this  is  known  as  panmerism.  Presum- 
ably this  power  of  assimilation,  growth  and  division  is  possessed 
by  particles  of  protoplasm  which  are  invisible  with  the  highest 
powers  of  our  microscopes,  though  it  is  probable  that  these  par- 
ticles are  much  larger  than  the  largest  molecules  known  to  chem- 
istry. The  smallest  particle  which  can  be  seen  with  the  most 
powerful  microscope  in  ordinary  light  is  about  250  ftp,  (millionths 
of  a  millimeter)  in  diameter.  The  largest  molecules  are  prob- 


130  Heredity  and  Environment 

ably  about  10  /U/A  in  diameter.  Between  these  molecules  and  the 
just  visible  particles  of  protoplasm  there  may  be  other  units  of 
organization.  These  hypothetical  particles  of  protoplasm  have 
been  supposed  by  many  authors  to  be  the  ultimate  units  of  assimi- 
lation, growth  and  division,  and  in  so  far  as  these  units  are  sup- 
posed to  be  the  differential  causes  of  hereditary  characters,  they 
are  known  as  inheritance  units. 

Inheritance  Units. — It  is  assumed  in  practically  all  theories  of 
heredity  that  the  "inheritance  material,"  or  the  germinal  proto- 
plasm, is  composed  of  ultra-microscopical  inheritance  units  which 
have  the  power  of  individual  growth  and  division  and  which  are 
capable  of  undergoing  many  combinations  and  dissociations  dur- 
ing the  course  of  development,  by  which  combinations  and 
dissociations  they  are  transformed  into  the  structures  of  the 
adult.  Various  names  have  been  given  to  these  units  by 
different  authors;  they  are  the  "physiological  units"  of  Herbert 
Spencer,  the  "gemmules"  of  Darwin,  the  "plastidules"  of  Els- 
berg  and  Haeckel,  the  "pangenes"  of  de  Vries,  the  "plasomes"  of 
Wiesner,  the  "idioblasts"  of  Hertwig,  the  "biophores"  and  "de- 
terminants" of  Weismann. 

With  the  publication  of  Weismann's  work  on  the  germplasm 
in  1892  speculation  with  regard  to  these  ultra-microscopic  units 
of  life  and  of  heredity  reached  a  climax  and  began  to  decline, 
owing  to  the  highly  speculative  character  of  the  evidence  as  to 
the«  existence,  nature  and  activities  of  such  units.  But  with  the 
rediscovery  of  Mendel's  principles  of  heredity  the  necessity  of 
assuming  the  existence  of  inheritance  units  of  some  kind  once 
more  became  evident,  and,  without  being  able  to  define  just  what 
such  units  are  or  just  how  they  behave,  modern  students  of  hered- 
ity assume  their  existence.  They  are  now  called  determiners  or 
factors  or  genes,  and  they  are  usually  thought  of  as  units  in  the 
germ  cells  which  condition  the  characters  of  the  developed  or- 
ganism, and  which  are  in  a  measure  independent  of  one  another; 
though  of  course  neither  they  nor  any  other  parts  of  a  cell  are 


The  Cellular  Basis  131 

really  independent  in  the  sense  that  they  can  exist  apart  from  one 
another.  They  are  to  be  thought  of  as  analogous  to  chemical 
radicals  which  are  never  independent  but  exist  only  in  combina- 
tion with  other  chemical  elements  in  the  form  of  molecules,  and 
yet  preserve  their  identity  in  many  different  combinations. 

It  is  certain  that  Mendelian  factors  are  not  to  be  regarded  as 
gemmules  or  the  germs  of  particular  characters.  There  is  not  a 
separate  factor  for  every  character,  and  factors  are  not  "repre- 
sentatives" or  "carriers"  of  characters.  They  are  the  differen- 
tial causes  of  particular  characters  just  as  in  the  compounds 
H2SO4  and  K2SO4  the  hydrogen  and  potassium  atoms  are  the 
differential  causes  of  the  properties  manifested  by  these  two 
substances. 

Location  of  Inheritance  Units. — If  there  are  inheritance  units, 
such  as  determiners  or  genes,  as  practically  all  students  of  heredity 
maintain,  they  must  be  contained  in  the  germ  cells,  and  it  becomes 
one  of  the  fundamental  problems  of  biology  to  find  out  where  and 
what  these  units  are.  There  are  many  evidences  that  these  genes 
are  located  in  the  chromatin  of  the  nucleus,  that  they  are  arranged 
in  a  linear  series  when  the  chromatin  takes  the  form  of  threads,  or 
chromosomes,  preparatory  to  cell  division,  that  in  the  division  of 
each  chromosome  every  gene  which  it  contains  is  also  divided 
and  that  daughter  chromosomes  and  daughter  genes  are  distrib- 
uted equally  to  the  daughter  cells  at  every  typical  cell  division 
(Figs.  6,  7,  8).  For  nearly  fifty  years  this  complex  process  of 
nuclear  division,  known  as  mitosis  or  karyokinesis,  has  been  recog- 
nized as  a  mechanism  for  the  equal  distribution  of  the  chromo- 
somes to  the  daughter  cells,  and  for  nearly  that  length  of  time  it 
has  been  suggested  that  the  inheritance  material  or  germplasm 
was  located  in  the  chromosomes,  but  only  within  recent  years  has 
critical  experimental  evidience  been  obtained  that  inheritance  units 
occupy  definite  positions  in  these  chromosomes.  With  this  ad- 
vance in  our  knowledge,  which  we  owe  chiefly  to  Morgan  and  his 
associates,  it  may  be  said  that  an  important  part,  at  least,  of  the 
"mechanism  of  heredity"  has  been  discovered. 


132  Heredity  and  Environment 

It  must  be  said  however  that  there  are  biologists  who  still  re- 
fuse to  believe  that  heredity  is  associated  with  any  particular  cell 
substance,  while  many  others  who  would  grant  this  are  not  yet 
ready  to  admit  that  there  are  particular  units  or  genes  which  are 
concerned  in  the  production  of  particular  characters.  However 
anyone  who  will  examine  at  first  hand  the  evidences  in  favor  of 
this  cannot  fail  to  be  impressed  with  its  importance,  and  no  one 
has  proposed  any  other  hypothesis  that  is  at  all  satisfactory.  But 
whether  we  assume  the  existence  of  these  units  or  not  we  know 
that  the  germ  cells  are  exceedingly  complex,  that  they  contain 
many  visible  units  such  as  chromosomes,  chromomeres,  plasto- 
somes  and  microsomes,  and  that  with  every  great  improvement 
in  the  microscope  and  in  microscopical  technique  other  structures 
are  made  visible  which  were  invisible  before,  and  whether  the  par- 
ticular hypothetical  units  just  named  are  invisible  or  not  seems  to 
be  a  matter  of  no  great  importance,  seeing  that,  so  far  as  the 
analysis  of  the  microscope  is  able  to  go,  there  are  in  all  proto- 
plasm differentiated  units  which  are  combined  into  a  system;  in 
short,  there  is  organization. 

6.  Heredity  and  Development. — The  germ  cells  are  individual 
organisms  and  after  the  fertilization  of  the  egg  the  new  individual 
thus  formed  remains  distinct  from  every  other  one.  Further- 
more, from  its  earliest  to  its  latest  stage  of  development  it  is  one 
and  the  same  organism ;  the  egg  is  not  one  being  and  the  embryo 
another  and  the  adult  a  third,  but  the  egg  of  a  human  being  is  a 
human  being  in  the  one-celled  stage  of  development,  and  the  char- 
acteristics of  the  adult  develop  out  of  the  egg  and  are  not  in  some 
mysterious  way  grafted  upon  it  or  transmitted  to  it. 

Parents  do  not  transmit  their  characters  to  their  offspring,  but 
their  germ  cells  in  the  course  of  long  development  give  rise  to 
adult  characters  similar  to  those  of  the  parents.  The  thing  which 
persists  more  or  less  completely  from  generation  to  generation 
is  the  organization  of  the  germ  cells  which  differentiate  in  similar 
ways  in  successive  generations  if  the  extrinsic  factors  of  develop- 
ment remain  similar. 


The  Cellular  Basis  133 

Definitions. — In  short,  heredity  may  be  defined  as  the  continuity 
from  generation  to  generation  of  certain  elements  of  germinal 
organization.  Heritage  is  the  sum  of  all  those  qualities  whiah  are 
determined  or  caused  by  this  germinal  organization.  Develop- 
ment is  progressive  and  coordinated  differentiation  of  the 
oosperm,  under  the  joint  influence  of  heredity  and  environment, 
by  which  it  is  transformed  into  the  adult  organisation.  Differ*- 
entiation  is  the  formation  and  localisation  of  many  different  kinds 
of  substances  out  of  the  germinal  substance,  of  many  different 
structures  and  functions  out  of  the  relatively  simple  structures 
and  functions  of  the  oosperm. 

This  germinal  organization  influences  not  merely  adult  charac- 
ters but  also  the  characters  of  every  stage  from  the  egg  to  the  adult 
condition.  For  every  inherited  character,  whether  embryonic  or 
adult,  there  is  some  germinal  basis.  In  the  last  analysis  the  causes 
of  heredity  and  development  are  problems  of  cell  structures  and 
functions,  problems  of  the  formation  of  particular  kinds  of 
germ  cells,  of  the  fusion  of  these  cells  in  fertilization,  and  of 
the  subsequent  formation  of  the  various  types  of  somatic  cells 
from  the  fertilized  egg  cell. 

B.    THE  GERM  CELLS 

Observations  and  experiments  on  developed  animals  and  plants 
have  furnished  us  with  a  knowledge  of  the  finished  products  of 
inheritance,  but  the  actual  stages  and  causes  of  inheritance,  the 
real  mechanisms  of  heredity,  are  to  be  found  only  in  a  study  of 
the  germ  cells  and  their  development.  Although  many  phenomena 
of  inheritance  have  been  discovered  in  the  absence  of  any  definite 
knowledge  of  the  mechanism  of  heredity,  a  scientific  explanation 
of  these  phenomena  must  wait  upon  the  knowledge  of  their  causes. 
In  the  absence  of  such  knowledge  it  has  been  necessary  to  formu- 
late theories  of  heredity  to  account  for  the  facts,  but  these  theo- 
ries are  only  temporary  scaffolding  to  bridge  the  gaps  in  our 
knowledge,  and  if  we  knew  all  that  could  be  known  about  the 


134  Heredity  and  Environment 

germ  cells  and  their  development  we  should  have  little  need  for 
theories.  In  the  first  chapter  we  looked  at  the  germ  cells  and 
their , development  from  the  outside,  as  it  were;  let  us  now  look 
inside  these  cells  and  study  their  minuter  structures  and  func- 
tions. 

Only  a  beginning  has  been  made  in  this  minute  study  of  the 
germ  cells  and  of  their  transformation  into  the  developed  animal, 
and  it  seems  probable  that  it  may  engage  the  attention  of  many 
future  generations  of  biologists,  but  nevertheless  we  have  come 
far  since  that  day  in  1875  when  Oscar  Hertwig  first  saw  the  ap- 
proach and  union  of  the  egg  and  sperm  nuclei  within  the  fertilized 
egg.  Indeed  so  rapid  has  been  the  advance  of  knowledge  in  this 
field  that  many  of  the  pioneers  in  this  work  are  still  active  in 
research. 

I.  Fertilization,  a.  Stimulus  to  Development. — The  development 
of  the  individual  may  be  said  to  begin  with  the  fertilization  of  the 
egg,  though  it  is  evident  that  both  egg  and  sperm  must  have  had 
a  more  remote  beginning,  and  that  they  also  have  undergone  a 
process  of  development  by  which  their  peculiar  characteristics  of 
structure  and  function  have  arisen, — a  subject  to  which  we  shall 
return  later.  But  the  developmental  processes  which  lead  to  the 
formation  of  fully  developed  ova  and  spermatozoa  come  to  a  full 
stop  before  fertilization  and  they  do  not  usually  begin  again 
until  a  spermatozoon  has  entered  an  ovum,  or  until  the  latter  has 
been  stimulated  by  some  other  outside  means. 

Parthenogenesis. — In  some  animals  and  plants,  eggs  may  de- 
velop regularly  without  fertilization,  the  stimulus  to  development 
being  supplied  by  certain  external  or  internal  conditions ;  in  other 
cases,  as  Loeb  discovered,  eggs  which  would  never  develop  if  left 
to  themselves  may  be  experimentally  stimulated  by  physical  or 
chemical  changes  in  the  environment,  so  that  they  undergo  regu- 
lar development.  The  development  of  an  egg  without  previous 
fertilization  is  known  as  parthenogenesis  or  virgin  reproduction; 
if  it  occurs  in  nature  it  is  natural  parthenogenesis,  if  in  experi- 


The  Cellular  Basis 


135 


ments  it  is  artificial  parthenogenesis.     Natural  parthenogensis  is 
relatively  rare  and  in  the  vast  majority  of  animals  and  plants  the 


FIG.  42.  DIAGRAMS  OF  THE  MATURATION  AND  FERTILIZATION  OF  THE  EGG 
OF  A  MOLLUSK  (Crepidula}.  A,  B,  First  maturation  division  (ist  Mat. 
Sp.).  C,  Second  maturation  division  (2d  Mat.  Sp.)  and  first  polar  body 
(ist  PB)  resulting  from  first  division.  $N,  Sperm  nucleus.  $C,  Sperm 
centrosome.  D,  Approach  of  sperm  nucleus  ($N)  and  sphere  ($S)  to 
egg  nucleus  (  9  A/")  and  sphere  (  $  S)  ;  the  second  polar  body  (2d  PB}  has 
been  formed  and  the  first  has  divided  {ist  PB}.  E,  Meeting  of  egg  and 
sperm  nuclei  and  origin  of  cleavage  centrosomes.  F,  First  cleavage  of 
egg  showing  direction  of  currents  in  the  cell. 


136  Heredity  and  Enmronment 

egg  does  not  begin  to  develop  until  a  spermatozoon  has  entered  it. 

b.  Union  of  Germplasms. — But  the  spermatozoon  not  only  stim- 
ulates the  egg  to  develop,  as  environmental  conditions  may  also 
do,  but  it  carries  into  the  egg  living  substances  which  are  of  great 
significance  in  heredity.  Usually  only  the  head  of  the  spermato- 
zoon enters  the  egg  (Fig.  4)  and  this  consists  almost  entirely  of 
nuclear  chromatin  (Fig.  4  D-H,  42  A-B)  ;  when  the  egg  has  ma- 
tured and  is  ready  to  be  fertilized  its  nucleus  also  consists  of  a 
small  mass  of  chromatin  (Fig.  42  C).  Both  of  these  condensed 
chromatic  nuclei  then  grow  in  size  and  become  less  chromatic  by 
absorbing  from  the  egg  a  substance  which  is  not  easily  stained 
by  dyes  and  hence  is  called  achromatin  (Figs.  4  I-L,  42  D-E). 
The  chromatin  then  appears  to  become  scattered  through  each 
nucleus  in  the  form  of  granules  or  threads  which  are  embedded  in 
the  achromatin;  this  is  the  condition  of  a  typical  "resting"  nu- 
cleus. It  is  evident  however  that  these  chromatin  granules  are 
not  scattered  broadcast  throughout  the  nucleus,  since  at  the  next 
mitosis  they  come  together  into  particular  chromosomes  similar  in 
every  way  to  the  chromosomes  of  the  previous  mitosis.  Probably 
the  chromosomes  preserve  their  identity  from  one  division  to  the 
next  either  in  the  form  of  chromosomal  vesicles  (Fig.  8,  p.  20) 
or  as  strings  of  granules.  The  spermatozoon  also  brings  into  the 
egg  a  centrosome  or  division  center,  around  which  an  aster  ap- 
pears consisting  of  radiating  lines  in  the  protoplasm  of  the  egg 
(Fig.  4F-7,  Fig.  42,  B-E). 

The  moment  that  the  spermatozoon  touches  the  surface  of  the 
egg  the  latter  throws  out  at  the  point  touched  a  prominence,  or 
reception  cone  (Fig.  4  A-E),  and  as  soon  as  the  head  of  the 
sperm  has  entered  this  cone  some  of  the  superficial  protoplasm  of 
the  egg  flows  to  this  point  and  then  turns  into  the  interior  of  the 
egg  in  a  kind  of  vortex  current.  Probably  as  a  result  of  this 
current  the  sperm  nucleus  and  centrosome  are  carried  deeper  into 
the  egg  and  finally  are  brought  near  to  the  egg  nucleus  (Fig.  42, 
D  and  E).  In  the  movements  of  egg  and  sperm  nuclei  toward 


The  Cellular  Basis 


FIG.  43.  FERTILIZATION  OF  THE  EGG  OF  THE  NEMATODE  WORM  Ascaris 
megalocephala.  $  N,  Egg  nucleus.  $  N,  Sperm  nucleus.  Arch,  Archiplasm. 
C,  Centrosome.  A,  B,  Approach  of  germ  nuclei.  C,  D,  Formation  of  two 
chromosomes  in  each  germ  nucleus.  E.  F,  Stages  in  the  division  of  the 
chromosomes  which  are  split  in  E  and  are  separating  in  F;  only  three 
of  the  four  chromosome  pairs  are  shown  in  F.  (From  Wilson  after 
Boveri.) 


138  Heredity  and  Environment 

each  other  it  is  probable  that  they  are  passively  carried  about  by 
currents  in  the  cytoplasm ;  the  entrance  of  the  sperm  serves  as  a 
stimulus  to  the  egg  cytoplasm  which  moves  according  to  its  pre- 
established  organization. 

2.  Cleavage  and  Differentiation. — When  the  sperm  nucleus 
has  come  close  to  the  egg  nucleus  the  sperm  centrosome  usually 
divides  into  two  minute  granules,  the  daughter  centrosomes,  which 
move  apart  forming  a  spindle  with  the  centrosomes  at  its  poles 
and  with  astral  radiations  running  out  from  these  into  the  cyto- 
plasm (Figs.  4,  42  F,  43  B-E). 

Egg  and  Sperm  Chromosomes. — At  the  same  time  the  chro- 
matin  granules  and  threads  in  the  egg  and  sperm  nuclei  take  the 
form  of  chromosomes,  and  at  this  stage  it  is  sometimes  possible 
to  see  that  each  chromosome  is  composed  of  a  series  of  granules, 
like  beads  on  a  string;  these  granules  are  the  chromomeres 
(Fig.  4  L).  The  number  of  chromosomes  is  constant  for  every 
species  and  race,  though  the  number  may  vary  in  different  spe- 
cies. In  the  thread  worm,  Ascaris  megalocephala,  there  are 
usually  two  chromosomes  in  the  egg  nucleus  and  two  in  the 
sperm  nucleus  (Fig.  43  D).  In  the  gastropod,  Crepidula  (Fig. 
45),  there  are  about  thirty  chromosomes  in  each  germ  nucleus  and 
sixty  in  the  two. 

Distribution  of  Chromosomes. — Then  the  spindle  and  asters 
grow  larger  and  the  nuclear  membrane  grows  thinner  and  finally 
disappears  altogether,  leaving  the  chromosomes  in  the  equator 
of  the  spindle  (Figs.  5  A,  6  F,  42  F,  and  43  F).  Each  of  the 
chromosomes  then  splits  lengthwise  into  two  equal  parts,  and  in 
the  splitting  of  the  chromosomes  it  is  sometimes  possible  to  see 
that  each  bead-like  chromomere  divides  through  its  middle.  The 
daughter  chromosomes  then  separate  and  move  to  opposite  poles  of 
the  spindle,  where  they  form  the  daughter  nuclei,  and  at  the  same 
time  the  cell  body  begins  to  divide  by  a  constriction  which  pinches 
the  cell  in  two  in  the  plane  which  passes  through  the  equator  of 
the  spindle  (Figs.  5,  7,  43  F,  45  B).  Finally  the  chromosomes 


The  Cellular  Basis 


139 


grow  in  size  by  the  absorption  of  achromatin  from  the  cell  body 
forming  the  chromosomal  vesicles  in  which  the  chromatin  takes 
the  form  of  threads  and  granules,  the  chromosomal  vesicles  unite 
to  form  the  daughter  nuclei  and  these  nuclei  come  back  to  a  "rest- 


9      *    * 


H 


FIG.  44.  MATURATION  AND  FERTILIZATION  OF  THE  EGG  OF  THE  MOUSE. 
A,  First  polar  body  and  second  maturation  spindle.  B,  second  polar  body 
and  maturation  spindle.  C,  Entrance  of  the  spermatozoon  into  the  egg. 
D-G,  Successive  stages  in  the  approach  of  egg  and  sperm  nuclei.  H,  for- 
mation of  chromosomes  in  each  germ  nucleus.  /,  First  cleavage  spindle 
showing  chromosomes  from  egg  and  sperm  on  opposite  sides  of  spindle. 
(After  Sobotta.) 


140  H'eredity  and  Environment 

ing"  stage  similar  to  that  with  which  the  division  began,  thus 
completing  the  "division  cycle"  of  the  cell  (Fig.  8). 

Identity  of  Chromosomes. — During  the  whole  division  cycle  it 
is  possible  in  a  few  instances  to  distinguish  the  chromosomes  of 
the  egg  from  those  of  the  sperm,  and  in  every  instance  where  this 
can  be  done  it  is  perfectly  clear  that  these  chromosomes  do  not 
fuse  together  nor  lose  their  identity,  but  that  every  chromosome 
splits  lengthwise  and  its  halves  separate  and  go  into  the  two 
daughter  cells  where  they  form  the  daughter  nuclei.  Each  of 
these  cells  therefore  receives  half  of  its  chromosomes  from  the  egg 
and  half  from  the  sperm.  Even  in  cases  where  the  individual 
chromosomes  are  lost  to  view  in  the  daughter  nuclei  those  nuclei 
are  sometimes  clearly  double,  one-half  of  each  having  come  from 
the  egg  chromosomes  and  the  other  half  from  the  sperm  chromo- 
somes (Fig.  45). 

At  every  subsequent  cleavage  of  the  egg  the  chromosomes  di- 
vide in  exactly  the  same  way  as  has  been  described  for  the  first 
cleavage.  Every  cell  of  the  developing  animal  receives  one-half 
of  its  chromosomes  from  the  egg  and  the  other  half  from  the 
sperm,  and  if  the  chromosomes  of  the  egg  differ  in  shape  or  in 
size  from  those  of  the  sperm,  as  is  sometimes  the  case  when  dif- 
ferent races  or  species  are  crossed,  these  two  groups  of  chromo- 
somes may  still  be  distinguished  at  advanced  stages  of  develop- 
ment. Where  the  egg  and  sperm  chromosomes  are  not  thus  dis- 
tinguishable it  may  still  be  possible  to  recognize  the  half  of  the 
nucleus  which  comes  from  the  egg  and  the  half  which  comes  from 
the  sperm  even  up  to  an  advanced  stage  of  the  cleavage  (Fig.  45). 

Distribution  of  Cytoplasm. — At  the  same  time  that  the  mater- 
nal and  paternal  chromosomes  are  being  distributed  with  such 
precise  equality  to  all  the  cells  of  the  developing  organism  the 
different  substances  in  the  cell  body  outside  of  the  nucleus  may 
be  distributed  very  unequally  to  the  cleavage  cells.  The  move- 
ments of  the  cytoplasm  of  the  egg,  which  began  with  the  flowing 
of  the  surface  layer  to  the  point  of  entrance  of  the  sperm,  and 


The  Cellular  Basis 


141 


which  continue  during  every  cleavage  of  the  egg,  lead  to  the  segre- 
gation of  different  kinds  of  plasms  in  different  parts  of  the  egg 


FIG.  45.  SUCCESSIVE  STAGES  IN  THE  CLEAVAGE  OF  THE  EGG  OF  A  MOLLUSK 
(Crepidula),  showing  the  separateness  of  the  male  and  female  chromo- 
somes ($ch,  9-c/O  and  of  the  male  and  female  halves  of  each  nucleus 


I42  Heredity  and  Environment 

and  to  the  unequal  distribution  of  these  substances  to  different 
cells  (Figs.  10,46,47). 

One  of  the  most  striking  cases  of  this  is  found  in  the  ascidian 
Styela,  in  which  there  are  four  or  five  substances  in  the  egg 
which  differ  in  color,  so  that  their  distribution  to  different  regions 
of  the  egg  and  to  different  cleavage  cells  may  be  easily  followed, 
and  even  photographed,  while  in  the  living  condition.  The  periph- 
eral layer  of  protoplasm  is  yellow  and  it  gathers  at  the  lower 
pole  of  the  egg,  where  the  sperm  enters,  forming  a  yellow  cap 
(Fig.  46,  i,  pi.).  This  yellow  substance  then  moves,  following  the 
sperm  nucleus,  up  to  the  equator  of  the  egg  on  the  posterior  side 
and  there  forms  a  yellow  crescent  extending  around  the  posterior 
side  of  the  egg  just  below  the  equator  (Fig.  46,  2-4).  On  the  an- 
terior side  of  the  egg  a  gray  crescent  is  formed  in  a  somewhat 
similar  manner  and  at  the  lower  pole  between  these  two  crescents 
is  a  slate  blue  substance,  while  at  the  upper  pole  is  an  area  of 
colorless  protoplasm.  The  yellow  crescent  goes  into  cleavage 
cells  which  become  muscles  and  mesoderm,  the  gray  crescent  into 
cells  which  become  nervous  system  and  notochord,  the  slate  blue 
substance  into  endoderm  cells  and  the  colorless  substance  into 
ectoderm  cells.  (Figs.  47  and  48;  see  also  Figs.  10  and  n.) 

Localization  of  Substances. — Thus  within  a  few  minutes  after 
the  fertilization  of  the  egg,  and  before  or  immediately  after  the 
first  cleavage,  the  anterior  and  posterior,  dorsal  and  ventral,  right 
and  left  poles  are  clearly  distinguishable,  and  the  substances  which 
will  give  rise  to  ectoderm,  endoderm,  mesoderm,  muscles,  noto- 
chord and  nervous  system  are  plainly  visible  in  their  character- 
istic positions. 

At  the  first  cleavage  of  the  egg  each  of  these  substances  is  di- 
vided into  right  and  left  halves  (Fig.  46,  5).  The  second  cleavage 
cuts  off  two  anterior  cells  containing  the  gray  crescent  from  two 
posterior  ones  containing  the  yellow  crescent  (Fig.  46,  6  and  Fig. 
47,  i).  The  third  cleavage  separates  the  colorless  protoplasm  in 
the  upper  hemisphere  from  the  slate  blue  in  the  lower  (Fig.  47, 


The  Cellular  Basis 


143 


1P.S. 


6 


FIG.  46.  SECTIONS  OF  THE  EGG  OF  Styela,  showing  maturation,  fertiliza- 
tion and  early  cleavage,  i  P.S.,  First  polar  spindle,  p.b.,  Polar  bodies; 
$  N,  sperm  nucleus,  $  N,  egg  nucleus,  p. I.,  peripheral  layer  of  yellow  pro- 
toplasm. Cr.,  Crescent  of  yellow  protoplasm.  As,  Aa,  Anterior  cells, 
B3,  Ba  Posterior  cells  of  the  4-cell  stage.  In  I  the  sperm  nucleus  and  cen- 
trosome  are  at  the  lower  pole  near  the  point  of  entrance ;  in  2  and  3  they 
have  moved  up  to  the  equator  on  the  posterior  side  of  the  egg;  in  4  the 
egg  and  sperm  nuclei  have  come  together  and  the  sperm  centrosome  has 
divided  and  formed  the  cleavage  spindle ;  in  5  the  egg  is  dividing  into  right 
and  left  halves ;  in  6  it  is  dividing  into  anterior  and  posterior  halves. 


144  Heredity  and  Environment 

2).  And  at  every  successive  cleavage  the  cytoplasmic  substances 
are  segregated  and  isolated  in  particular  cells,  and  in  this  way 
the  cytoplasm  of  the  different  cells  comes  to  be  unlike  (Figs.  47 
and  48).  When  once  partition  walls  have  been  formed  between 
cells  the  substances  in  the  different  cells  are  permanently  sepa- 
rated so  that  they  can  no  longer  commingle. 

What  is  true  of  Styela  in  this  regard  is  equally  true  of  many 
other  ascidians,  as  well  as  of  Amphioxus  and  of  the  frog  (Figs. 
9,  10,  n),  though  the  segregation  of  substances  and  the  differ- 
entiation of  cells  are  not  so  evident  in  the  last  named  animals  be- 
cause these  substances  are  not  so  strikingly  colored.  Indeed  the 
segregation  and  isolation  of  different  protoplasmic  substances 
in  different  cleavage  cells  occurs  during  the  cleavage  of  the  egg 
in  all  animals,  though  such  differentiations  are  much  more  marked 
in  some  cases  than  in  others. 

This  same  type  of  cell  division,  with  equal  division  of  the 
chromosomes  and  more  or  less  unequal  division  of  the  cell  body, 
continues  long  after  the  cleavage  stages,  indeed  throughout  the 
entire  period  of  embryonic  development.  Sometimes  the  division 
of  the  cell  body  is  equal,  the  daughter  cells  being  alike;  sometimes 
it  is  unequal  or  differential,  but  always  the  division  of  the  chro- 
mosomes is  equal  and  non-differential.  When  once  the  various 
tissues  have  been  differentiated  the  further  divisions  in  these 
tissue  cells  are  usually  non-differential  even  in  the  case  of  the 
cell  bodies. 

Significance  of  Cleavage. — There  can  be  no  doubt  that  this  re- 
markably complicated  process  of  cell  division  has  some  deep 
significance;  why  should  a  nucleus  divide  in  this  peculiarly  in- 
direct manner  instead  of  merely  pinching  in  two,  as  was  once 
supposed  to  be  the  rule  ?  What  is  the  relation  of  cell  division  to 
embryonic  differentiation  ?  In  this  process  of  mitosis,  or  indirect 
cell  division,  two  important  things  take  place:  (i)  Each  chro- 
mosome, chromomere  and  centrosome  is  divided  exactly  into  two 
equal  parts  so  that  each  daughter  structure  is  at  the  time  of  its 


The  Cellular  Basis  145 

formation  quantitatively  and  qualitatively  precisely  like  its  mother 
structure.  (2)  Accompanying  the  formation  of  radiations,  which 
go  out  from  the  centrosomes  into  the  cell  body,  diffusion  currents 
are  set  up  in  the  cytoplasm  which  lead  to  the  localization  of  dif- 
ferent parts  of  the  cytoplasm  in  definite  regions  of  the  cell,  and 
this  cytoplasmic  localization  is  sometimes  of  such  a  sort  that  one 
of  the  daughter  cells  may  contain  one  kind  of  cell  substance  and 
the  other  another  kind. 

Cytoplasm  Differentiates,  Nuclei  Do  Not. — Thus  while  mitosis 
brings  about  a  scrupulously  equal  division  of  the  elements  of 
the  nucleus,  it  may  lead  to  a  very  unequal  and  dissimilar  division 
of  the  cytoplasm.  In  this  is  found  the  significance  of  mitosis, 
and  it  suggests  at  once  that  the  nucleus  contains  non-differen- 
tiating material,  viz.,  the  idioplasm  or  germplasm,  which  is  char- 
acteristic of  the  race  and  is  carried  on  from  cell  to  cell  and  from 
generation  to  generation ;  whereas  the  cell  body  contains  the  dif- 
ferentiating substance,  the  personal  plasm  or  somatoplasm,  which 
gives  rise  to  all  the  differentiations  of  cells,  tissues  and  organs 
in  the  course  of  ontogeny. 

Weismann  supposed  that  the  mitotic  division  of  the  chromo- 
somes during  development  was  of  a  differential  character,  the 
daughter  chromosomes  differing  from  each  other  at  every  dif- 
ferential division  in  some  constant  and  characteristic  way,  and  that 
these  differentiations  of  the  chromosomes  produced  the  charac- 
teristic differentiations  of  the  cytoplasm  which  occur  during 
development.  But  there  is  not  a  particle  of  evidence  that  the  ordi- 
nary division  of  chromosomes  is  differential;  on  the  contrary, 
there  is  the  most  complete  evidence  that  their  division  is  remark- 
ably equal  both  quantitatively  and  qualitatively.  If  daughter 
chromosomes  and  nuclei  ever  become  unlike,  as  they  sometimes  do, 
this  unlikeness  occurs  long  after  division  and  is  probably  the 
result  of  the  action  of  different  kinds  of  cytoplasm  upon  the 
nuclei,  as  is  true  for  example,  in  the  differentiation  of  the  chn>- 
mosomes  in  the  somatic  cells  as  contrasted  with  the  germ  cells  of 


146 


Heredity  and  Environment 


Ascaris  (Fig.  49).  In  this  case  Boveri  has  shown  that  the  nuclei 
and  chromosomes  of  germ  cells  and  of  somatic  cells  are  at  first 
alike  whereas  the  cytoplasm  in  these  cells  is  unlike;  later  the 
nuclei  and  chromosomes  of  these  two  kinds  of  cells  become  unlike, 


FIG.  47.  CLEAVAGE  OF  THE  EGG  OF  Styela,  showing  distribution  of  the 
yellow  protoplasm  (stippled)  and  of  the  clear  and  gray  protoplasm  to 
the  various  cells,  each  of  which  bears  a  definite  letter  and  number. 


The  Cellular  Basis 


147 


owing  probably  to  the  peculiarities  of  the  cytoplasm  of  these  cells. 
These  nuclear  differentiations  are  caused  by  differential  divisions 
of  the  cytoplasm  and  not  of  the  nucleus.  But  while  the  chromo- 


FIG.  48.  GASTRULA  AND  LARVA  OF  Styela,  showing  the  cell  lineage  of  vari- 
ous organs,  and  the  distribution  of  the  different  kinds  of  protoplasm  to 
these  organs.  Muscle  cells  are  shaded  by  vertical  lines,  mesenchyme  by 
horizontal  lines,  nervous  system  and  chorda  by  stipples. 


148  Heredity  and  Environment 

somes  themselves  divide  equally,  other  portions  of  the  nucleus 
may  not  do  so.  Nuclear  achromatin  and  oxychromatin,  like  the 
cytoplasm,  may  divide  unequally  and  differentially,  and  this  is 
probably  a  prime  factor  in  development. 

On  the  other  hand,  the  differential  division  of  the  cytoplasm  is 
a  regular  and  characteristic  feature  of  ontogeny ;  indeed,  the  seg- 
regation and  isolation  of  different  kinds  of  cytoplasm  in  differ- 
ent cells  is  one  of  the  most  important  functions  of  cell  division 
during  development.  Thus  we  find  in  the  division  apparatus  of 
the  cell  a  mechanism  for  the  preservation  in  unaltered  form  of  the 
species  plasm  or  germplasm  of  the  nucleus,  and  for  the  progres- 
sive differentiation  of  the  personal  plasm  or  somatoplasm  of  the 
cell  body. 

3.  The  Origin  of  the  Sex  Cells. — The  sex  cells  are  among  the 
latest  of  all  cells  of  a  developing  animal  to  reach  maturity,  and  yet 
they  may  be  among  the  earliest  to  make  their  appearance.  Every 
sex  cell,  like  every  other  type  of  cell,  is  a  lineal  descendant  of 
the  fertilized  egg  (Fig.  41),  but  the  period  at  which  the  sex  cells 
become  visibly  different  from  other  cells  varies  from  the  first 
cleavage  of  the  egg  in  some  species  to  a  relatively  advanced  stage 
of  development  in  others. 

(a)  The  Division  Period.  Oogonia  and  Spermatogonia. — When 
the  primitive  sex  cells  are  first  distinguishable  they  differ  from 
other  cells  only  in  the  fact  that  they  are  less  differentiated;  they 
have  relatively  larger  nuclei  and  smaller  cell  bodies,  a  condition 
which  is  indicative  of  little  differentiation  of  the  cell  body  since 
the  products  of  differentiation  such  as  fibres,  secretions,  etc.,  swell 
the  size  of  the  cell  body  but  do  not  contribute  to  the  growth  of 
the  nucleus.  These  primitive  sex  cells  or  gonia  divide  repeatedly, 
but  the  oogonia  grow  more  rapidly  and  divide  less  frequently 
than  the  spermatogonia.  As  a  result  of  this  difference  in  the 
rate  of  growth  and  division  the  spermatogonia  become  much 
smaller  and  immensely  more  numerous  than  the  oogonia.  This 


The  Cellular  Basis 


149 


period  in  the  genesis  of  the  sex  cells  is  known  as  the  division 
period  (Fig.  41). 


FIG.  49.  DIFFERENTIATION  OF  GERM  CELLS  AND  SOMATIC  CELLS  IN  THE 
EGG  OF  Ascaris.  A  and  B,  Second  cleavage  division  showing  that  the  chro- 
mosomes remain  entire  in  the  lower  cell,  which  is  in  the  line  of  descent  of 
the  sex  cells  ("germ  track"),  but  that  they  throw  off  their  ends  and  break 
up  into  small  granules  in  the  upper  cells,  which  become  somatic  cells. 
C,  4-cell  stage,  the  nuclei  in  the  upper  (somatic)  cells  being  small  and  the 
ends  of  the  chromosomes  remaining  as  chromatic  masses  in  the  cell  body 
outside  of  the  nuclei,  while  the  nuclei  in  the  lower  cells  are  much  larger 
and  contain  all  of  their  chromatin.  D,  Third  nuclear  division,  showing  the 
somatic  differentiation  of  the  chromosomes  in  all  the  cells  except  the  lower 
right  one,  which  alone  is  in  the  germ  track  and  will  ultimately  give  rise  to 
sex  cells.  (After  Boveri.) 


Heredity  and  Environment 


(b)  The  Growth  Period.  Oocytes  and  Spermatocytes. — This 
period  of  rapid  cell  division  is  followed  by  a  period  of  growth 
without  division  during  which  the  developing  sex  cells  are  called 
primary  oocytes  or  spermatocytes.  This  growth  period  may  be 
very  long  in  the  case  of  the  oocytes,  lasting,  for  example,  in  the 
human  female  from  the  time  of  birth  to  the  end  of  the  reproduc- 
tive period ;  during  this  long  time  the  oocytes  in  the  ovary  prob- 
ably never  divide,  there  are  as  many  of  them  at  birth  as  at  any 
later  time;  during  this  period  of  growth  the  ovarian  egg  becomes 
relatively  large, — in  some  animals,  e.g.,  birds,  the  largest  of  all 


FIG.  50.  DIFFERENT  STAGES  IN  THE  DEVELOPMENT  OF  THE  EGG  OF  THE 
RABBITT.  A,  At  the  beginning  of  the  growth  period  showing  slender 
chromatic  threads  in  the  nucleus.  B,  Later  stage  in  which  these  threads 
ball  up  and  parallel  threads  conjugate  forming  the  shorter,  thicker  thread 
shown  in  C. — D  and  E,  Later  stages  showing  pairs  of  chromosomes  due 
to  conjugation.  F,  Later  stage  in  which  the  distinctness  of  the  chromo- 
somes is  temporarily  lost.  (After  Winiwarter.) 


The  Cellular  Basis  151 

cells.  The  growth -period  of  a  spermatocyte  lasts  for  a  briefer 
time  than  does  that  of  an  oocyte  so  that  the  former  remains 
relatively  small  (Fig.  41). 

Sy  nap  sis. — All  of  the  cell  divisions  which  take  place  during 
the  division  period  are  of  the  usual  kind,  in  which  every  chromo- 
some splits  lengthwise  into  two  and  the  two  halves  then  separate 
and  move  to  opposite  poles  of  the  spindle  where  they  swell  up 
into  chromosomal  vesicles  and  form  the  daughter  nuclei,  as  is 
shown  in  Figs.  7,  and  43.  But  during  the  growth  period  of  the 
oocytes  and  spenmatocytes  the  chromosomes  form  a  closely 
wound  coil  of  long  chromatin  threads  (Fig.  50  A  and  B),  and 
when  these  threads  uncoil  later  it  is  seen  that  the  chromosomes 
have  united  in  pairs  (Figs.  50  D  and  E,  50  a,  51  B,  52  B)  ;  this 
process  is  known  as  synapsis,  or  the  conjugation  of  the  chromo- 
somes, and  there  is  evidence  that  one  member  of  each  synaptic  pair 
is  derived  from  the  father,  and  the  other  from  the  mother.  The 
union  of  these  chromosomes  is  a  temporary  one  and  is  not  so  close 
that  they  lose  their  identity.  By  this  union  of  the  chromosomes 
into  pairs  the  number  of  separate  chromosomes  is  reduced  to  half 
the  normal  number ;  if  there  are  usually  4  chromosomes,  as  in  As- 
caris,  they  are  reduced  to  2  pairs ;  if  48  chromosomes,  as  in  man, 
there  are  24  of  these  pairs. 

Conjugation  of  Homologous  Chromosomes. — In  the  conjuga- 
tion of  the  chromosomes  it  is  plain  that,  generally  speaking, 
those  chromosomes  unite  which  are  similar  in  shape  and  size; 
big  chromosomes  unite  with  big  ones,  little  ones  with  little  ones, 
and  those  of  peculiar  shape  with  others  of  similar  shape  (Figs. 
50  a,  51  B,  52  B,  54,  58).  It  is  probable  that  the  two  members  of 
a  pair  of  conjugating  chromosomes  are  homologous  not  merely  in 
shape  and  size  but  also  in  function,  though  this  homology  does  not 
amount  to  identity.  These  homologous  chromosomes  may  be  com- 
pared to  the  fingers  of  the  two  hands ;  each  digit  differs  from  every 
other  one  but  the  thumb,  index  finger  and  other  fingers  of  the 
right  hand  are  homologous  but  not  identical  with  the  correspond- 


152 


Heredity  and  Environment 


FIG.  50  a.  SYN APSIS  (CONJUGATION)  OF  CHROMOSOMES  in  the  grasshopper 
Phrynotettix.  A.  At  the  left,  Telophase  of  Spermatogonium  showing 
chromosomes  in  nucleus,  among  them  X  and  a  pair  B.  At  the  right,  are  12 
pairs  of  B  chromosomes  in  synapsis,  each  pair  from  a  different  animal, 
and  one  member  of  each  pair  from  the  father,  the  other  from  the  mother. 
Homologous  chromomeres  (granules  I,  2,  3,  4,  5)  are  shown  in  each 
chromosome.  B  similar  stages  in  chromosome  pair  A.  Some  of  the 
chromosomes  in  the  middle  show  a  "secondary"  longitudinal  split  and  a 
"crossing  over"  (?)  of  the  halves.  C.  Tetrads  (conjugated  chromosomes) 
of  pair  B  formed  by  shortening  and  thickening  of  the  chromosome  pairs 
and  by  the  appearance  of  the  "secondary"  split.  (After  Wenrich.) 

ing  digits  of  the  left  hand,  and  the  conjugation  of  homologous 
chromosomes  may  be  compared  to  the  placing  together  of  the 
two  hands* so  that  homologous  digits  come  together. 

In  some  instances  it  can  be  proved  that  one  member  of  each 
conjugating  pair  of  chromosomes  comes  from  one  parent  and  the 
other  from  the  other  parent,  and  it  is  probable  that  this  is  always 


The  Cellular  Basis  153 

true.  In  every  cell  of  every  individual  which  has  developed 
from  a  fertilized  egg  there  are  two  full  sets  of  chromosomes,  one 
of  which  came  from  the  sperm  and  the  other  from  the  egg;  but 
when  this  individual  in  its  turn  produces  germ  cells  homologous 
chromosomes  of  each  set  unite  in  pairs,  side  by  side,  during  the 
growth  period.  This  again  may  be  compared  to  the  union  of 
the  two  hands,  the  right  digits,  for  example,  representing  the 
paternal  and  the  left  the  maternal  chromosomes  which  come  to- 
gether in  homologous  pairs,  corresponding  joints  lying  opposite 
each  other,  as  corresponding  genes  in  the  chromosomes  lie  oppo- 
site each  other. 

These  synaptic  pairs  are  the  bivalent  chromosomes,  and  in  ad- 
dition to  showing  the  line  of  junction  by  which  they  are  united 
they  frequently  show  a  longitudinal  split  through  the  middle 
of  each  chromosome  and  at  right  angles  to  the  line  of  junction. 
It  thus  happens  that  these  bivalent  chromosomes  are  frequently 
four-parted  and  such  four-parted  chromosomes  are  known  as 
tetrads  (Figs.  51  B,  52  B,  C). 

(c)  The  Maturation  Period. — Finally  at  the  close  of  the 
growth  period  both  oocyte  and  spermatocyte  undergo  two  peculiar 
divisions,  one  following  immediately  after  the  other,  which  are 
unlike  any  other  cell  divisions.  These  are  known  as  the  first  and 
second  maturation  divisions  and  they  are  the  last  divisions  which 
take  place  in  the  formation  of  the  egg  and  sperm. 

Reduction  Division. — In  one  or  the  other  of  these  two  matura- 
tion divisions  the  pairs  of  chromosomes  separate  along  the  line 
of  junction,  one  member  of  each  pair  going  to  one  pole  of  the 
spindle  and  the  other  to  the  other  pole,  so  that  in  each  of  the 
daughter  cells  thus  formed  only  a  single  set  of  chromosomes  is 
present  (Figs.  51  C,  D,  54)  ;  but  since  the  position  of  the  pairs 
of  chromosomes  in  the  spindle  is  a  matter  of  chance  it  rarely 
happens  that  all  the  paternal  chromosomes  go  to  one  pole  and  all 
the  maternal  ones  to  the  other;  thus  each  of  the  sex  cells  comes 
to  contain  a  complete  set  of  chromosomes,  though  particular  indi- 


154 


Heredity  and  Environment 

A.         ^-rrr>^  -B 


FIG.  51.  SPERMATOGENESIS  OF  A  NEMATODE  WORM  (Ancyracanthus} . 
A,  Chromosomes  of  sperm  mother  cell,  n  in  number,  before  their  union 
into  pairs.  B,  early  stage  of  first  maturation  division;  10  of  the  chromo- 
somes have  united  into  5  pairs  and  each  of  these  has  split  lengthwise;  I 
chromosome  remains  unpaired.  C,  First  maturation  division  after  the  5 
pairs  of  chromosomes  have  pulled  apart;  the  unpaired  chromosome  is 
going  entire  to  one  pole  of  the  spindle.  D,  Two  cells  resulting  from  this 
division,  one  containing  5  and  the  other  6  chromosomes.  E,  Four  cells  re- 
sulting from  the  division  of  the  two  cells  like  D,  in  which  every  chromo- 
some has  split  into  two  so  that  two  of  the  cells  contain  5  and  two  contain  6 
chromosomes.  F ,  two  of  these  cells  changing  into  spermatozoa,  one  con- 
taining 5  and  the  other  6  chromosomes.  (After  Mulsow.) 


The  Cellular  Basis 
A  B 


155 


FIG.  52.  OOGENESIS  OF  A  NEMATODB  WORM  (Ancyracanthiis) .  A,  Egg 
mother  cell  containing  12  chromosomes  before  their  union  into  pairs.  B, 
Early  stage  of  first  maturation  division ;  all  the  chromosomes  have  united 
into  6  pairs,  and  all  but  one  of  these  has  split  in  two  so  that  the  pairs  are 
really  four-parted  (tetrads).  C,  The  six  tetrads  in  the  first  maturation 
division.  D,  Egg  containing  6  chromosomes,  after  both  first  and  second 
maturation  divisions;  the  eliminated  chromosomes  are  shown  as  the  polar 
bodies  at  the  margin  of  the  egg.  E  and  F,  Eggs  after  fertilization;  the 
egg  nucleus  is  above  and  contains  6  chromosomes,  the  sperm  nucleus  is 
below  and  contains  5  chromosomes  in  one  case  and  6  in  the  other;  in  the 
former  case  the  egg  becomes  a  male  with  n  chromosomes,  in  the  latter 
a  female  with  12  chromosomes.  (After  Mulsow.) 


156  Heredity  and  Environment 

vidual  chromosomes  may  have  come  from  the  father  while  others 
have  come  from  the  mother.  There  is  reason  to  believe  that 
homologous  chromosomes  show  general  resemblances  but  indi- 
vidual differences,  ,antf  consequently  when  the  members  of  each 
pair  separate  and  go  into  the  sex  cells  these  cells  differ  among 
themselves  because  the  individual  chromosomes  in  different  cells 
are  not  the  same  in  hereditary  value  (Fig.  58). 

Again  this  is  comparable  to  the  separation  of  the  digits  of  the 
two  hands  after  these  have  been  placed  together  in  homologous 
pairs,  except  that  all  the  right  digits  must  go  with  the  right  hand, 
all  the  left  ones  with  the  left  hand  since  they  are  permanently  at- 
tached to  the  hands.  But  the  homologous  chromosomes  are  free 
and  each  chromosome  of  a  pair  may  separate  to  the  right  or  to 
the  left  as  the  digits  might  do  if  they  were  severed  from  the 
hands.  Then  each  digit  of  a  pair  could  separate  either  to  the 
right  or  to  the  left  and  it  would  rarely  happen  that  all  the  right 
hand  ones  would  go  in  one  direction  and  all  the  left  hand  ones 
in  the  other,  but  since  the  members  of  each  pair  could  separate  in 
two  directions  and  since  there  are  five  pairs,  the  possible  number 
of  different  combinations  after  separation  would  be  (2)5  or  32, 
and  yet  in  each  of  these  combinations  the  full  set  of  digits  from 
the  thumb  to  the  little  finger  would  be  present.  Finally  the  fer- 
tilization of  the  egg  may  be  likened  to  the  game  of  "bean  por- 
ridge" in  which  different  hands  (gametes)  each  with  its  full  set 
of  digits  (chromosomes)  are  .struck  together  thus  making  new 
c6mbinations  of  digits  (chromosomes). 

In  this  way  the  number  of  chromosomes  in  the  mature  egg  or 
sperm  comes  to  be  one-half  the  number  present  in  other  kinds 
of  cells,  and  when  the  egg  and  sperm  unite  in  fertilization  the 
whole  number  is  again  restored.  The  double  set  of  chromosomes 
is  known  as  the  diploid  number,  the  single  set  as  the  haploid 
number,  and  the  maturation  division  in  which  this  reduction  from 
the  double  to  the  single  set  takes  place  is  the  reduction  division. 
It  is  generally  held  that  this  reduction  takes  place  in  the  first  of  the 


The  Cellular  Basis  157 

two  maturation  divisions  (Fig.  51  C,  D),  and  that  the  second  of 
these  divisions  is  like  an  ordinary  mitosis  in  that  each  chromo- 
some splits  into  two  and  the  halves  move  apart,  such  a  division 
being  known  as  an  equation  division  (Fig.  51  E),  but  it  is  pos- 
sible that  some  chromosome  pairs  undergo  an  equation  division 
in  the  first  maturation  mitosis  and  a  reduction  division  in  the  sec- 
ond, while  other  chromosome  pairs  may  reverse  this  order. 

It  is  an  interesting  fact  that  long  before  the  reduction  of  chro- 
mosomes had  been  actually  seen  Weismann  maintained  on  theoret- 
ical grounds  that  such  a  reduction  must  occur,  otherwise  the 
number  of  chromosomes  would  double  in  every  generation,  and 
he  held  that  this  reduction  must  take  place  in  one  of  the  matura- 
tion divisions;  this  hypothesis  of  Weismann's  is  now  an  estab- 
lished fact. 

Mature  Egg  and  Sperm. — As  the  result  of  these  two  matura- 
tion divisions  four  cells  are  formed  from  each  cell  (spermato- 
cyte  or  oocyte)  of  the  growth  period.  In  the  spermatogenesis 
each  of  these  four  cells  is  transformed  into  a  functional  sperma- 
tozoon (Figs.  41,  51  F)  by  the  condensation  of  the  nucleus  into 
the  sperm  head  and  the  outgrowth  of  the  centrosome  and  cyto- 
plasm to  form  the  tail.  In  the  oogenesis  only  tfne  of  these  four 
cells  becomes  a  functional  egg  while  the  other  three  are  small 
rudimentary  eggs  which  are  called  polar  bodies  and  which  take 
no  further  part  in  development  (Figs.  41,  42  C-F).  The  fertili- 
zation of  the  egg  usually  takes  place  coincidently  with  the  for- 
mation of  the  polar  bodies,  and  so  we  come  back  once  more  to 
the  stage  from  which  we  started,  thus  completing  the  life  cycle. 

C.    SEX  DETERMINATION 

In  the  formation  of  the  sex  cells  one  can  distinguish  at  an  early 
stage  differences  between  the  larger  oogonia  and  the  smaller  and 
more  numerous  spermatogonia ;  this  difference  is  the  first  visible 
distinction  in  the  development  of  the  two  sexes.  In  the  case  of 
the  human  embryo  this  distinction  can  be  made  as  early  as  the 


158  Heredity  and  Environment 

fifth  week,  and  it  is  evident  that  the  real  causes  of  this  differ- 
ence must  be  found  at  a  still  earlier  period  of  development. 

The  cause  of  sex  has  been  a  favorite  subject  of  speculation  for 
thousands  of  years.  Hundreds  of  hypotheses  have  been  advanced 
to  explain  this  perennially  interesting  phenomenon.  The  causes 
of  sex  determination  have  been  ascribed  to  almost  every  possible 
external  or  internal  influence  and  the  world  is  full  of  people  who 
think  they  have  discovered  by  personal  experience  just  how  sex 
is  determined.  Unfortunately  these  hypotheses  and  rules  are  gen- 
erally founded  upon  a  few  observations  of  selected  cases.  Since 
there  are  only  two  sexes  the  chances  are  that  any  hypothesis 
will  be  right  half  the  time,  and  if  only  one  forgets  the  failures 
of  a  rule  and  remembers  the  times  when  it  holds  good  it  is  possi- 
ble to  believe  in  the  influence  of  food  or  temperature  or  age,  of 
war  or  peace  or  education  on  the  relative  numbers  of  the  sexes, 
or  on  almost  any  other  thing.  By  statistics  it  has  been  shown 
that  each  of  these  things  influences  the  sex  ratio,  and  by  more 
extensive  statistics  it  has  been  proved  that  they  do  not. 

i.  Chromosomal  Determination:  XO  Type. — This  was  the 
condition  regarding  the  causes  of  sex  determination  which  pre- 
vailed up  to  the  year  1902.  Immediately  preceding  that  year  it 
had  been  found  that  two  kinds  of  spermatozoa  were  formed  in 
equal  numbers  in  certain  insects;  one  of  these  kinds  contained  a 
peculiar  "accessary"  or  "odd"  chromosome,  and  the  other  lacked 
it.  The  manner  in  which  these  two  types  of  spermatozoa  were 
formed  had  been  carefully  worked  out  by  several  investigators 
without  any  suspicion  of  the  real  significance  of  the  facts.  It 
was  shown  that  an  uneven  number  of  chromosomes  might  be 
present  in  the  spermatogonia  of  certain  insects  and  that  when 
maternal  and  paternal  chromosomes  united  in  pairs  in  synapsis 
one  "odd"  chromosome  was  left  without  a  mate  (Fig.  51  B). 
Later,  in  the  reduction  division,  when  the  synaptic  pairs  sep- 
arated, the  odd  chromosome  werit  entire  into  one  of  the  daughter 
cells,  and  the  spermatozoa  formed  from  this  cell  contained  one 


The  Cellular  Basis 


159 


chromosome  more  than  those  formed  from  the  other  daughter  cell 
(Fig.  51  C-F). 

Chiefly  because  these  two  kinds  of  spermatozoa  occur  in  equal 
numbers  McClung  in  1902  concluded  that  this  accessory  chromo- 
some was  a  sex-determinant.  In  1905  Wilson  discovered  in  a 
number  of  bugs  that  while  there  were  two  types  of  spermatozoa, 
one  of  which  contained  and  the  other  lacked  the  accessory 
chromosome,  there  was  only  one  type  of  egg,  since  every  egg  con- 
tained the  accessory  chromosome,  and  he  pointed  out  that  if  an 
egg  were  fertilized  by  a  sperm  containing  an  accessory,  two  ac- 
cessories would  be  present  in  the  zygote,  this  being  the  condition 
of  the  female,  while  if  it  were  fertilized  by  a  sperm  without  an 
accessory  there  would  be  present  in  the  zygote  only  the  accessory 


Pratenor    Type 


Oocyto 


Bmrmatooyte      Kedvcti6n 


Sp/armaAids 


FIG.  53.  DIAGRAMS  OF  SEX  DETERMINATION  IN  THE  BUG,  Protenor.  The 
oocyte  contains  6  chromosomes  and  the  spermatocyte  5  chromosomes 
which  are  not  yet  united  into  synaptic  pairs;  the  "sex"  chromosomes  are 
shown  in  black,  two  are  present  in  the  oocyte,  but  only  one  in  the  sperma- 
tocyte. In  the  reduction  division  the  synaptic  pairs  separate,  giving  rise  to 
two  types  of  spermatids,  one  of  which  has  the  sex  chromosome  and  the 
other  lacks  it ;  all  ova  are  alike  in  this  regard.  If  an  egg  is  fertilized  by 
a  sperm  without  the  sex  chromosome  a  male  results;  if  fertilized  by  a 
sperm  containing  the  sex  chromosome  a  female  results.  (After  Wilson 
with  modifications.) 


i6o 


Heredity  and  Environment 


derived  from  the  egg  (Fig.  52  E  and  F,  Fig.  53).  This  "acces- 
sory" chromosome  was  therefore  called  the  "sex  determining"  or 
merely  the  "sex"  chromosome  and  was  designated  by  the  letter  X; 
consequently  its  double  occurrence  in  the  female  was  indicated  by 
XX;  its  single  occurrence  in  the  male  by  XO,  the  O  standing  for 
zero  or  no  chrompsome. 

XY  Type. — In  other  cases  Miss  Stevens  as  well  as  Wilson  dis- 
covered that  two  accessory  chromosomes,  differing  in  size,  might 
be  present  in  the  male  whereas  in  the  female  they  are  of  equal 
size  (Fig.  54).  In  such  cases  two  types  of  spermatozoa  are  pro- 
duced in  equal  numbers,  one  containing  a  large  and  the  other  a 
small  accessory  chromosome,  whereas  every  egg  contains  one 
large  accessory  chromosome.  If  such  an  egg  is  fertilized  by  a 
sperm  containing  a  large  accessory  (the  X  chromosome)  it  gives 
rise  to  a  female  with  the  formula  XX,  if  by  a  sperm  containing 


Oocyte 


Fertilized 


FIG.  54.  DIAGRAMS  OF  SEX  DETERMINATION  IN  THE  BEETLE,  Tenebrio, 
showing  5  synaptic  pairs  of  chromosomes  (there  are  actually  10  pairs)  ; 
in  the  oocyte  the  members  of  each  pair  are  equal  in  size;  in  the  sper- 
matocyte  the  members  of  one  pair  are  unequal.  These  pairs  separate  in 
the  reduction  division  giving  rise  to  two  types  of  spermatozoa  and  one  type 
of  ova;  eggs  fertilized  by  one  type  of  sperm  give  rise  to  females,  those 
fertilized  by  the  other  type  give  rise  to  males.  (After  Stevens  with 
modifications.) 


The  Cellular  Basis 


161 


a  small  accessory  (the  Y  chromosome)  it  gives  rise  to  a  male  with 
the  formula  XY  (Fig.  54). 

In  other  animals  one  may  not  be  able  to  distinguish  separate 
X  or  Y  chromosomes  and  yet  such  structures  may  be  joined  to 
one  or  two  ordinary  chromosomes.  This  is  the  case  in  the  thread 
worm,  Ascaris  (Fig.  55),  where  two  such  accessory  elements  are 
present  in  the  female,  each  being  joined  to  the  end  of  an  ordinary 
chromosome,  whereas  in  the  male  only  one  such  element  is  pres- 
ent. Here  also  two  classes  of  spermatozoa  are  found  one  with 
and  the  other  without  the  accessory  element,  whereas  all  ova  have 
this  element,  and  in  this  case  also  sex  is  probably  determined  by 
the  type  of  spermatozoon  which  enters  the  egg  (Fig.  55). 

Recurring  to  the  comparison  of  digits  and  chromosomes,  the 
chromosomal  theory  of  sex  determination  may  be  illustrated 
by  assuming  that  all  males  have  lost  a  particular  digit,  say  the 

Atcaris     Type 


Jterf  action 
Division 


Mature  Egg  and 
Polar  U 


W    ®r -^5 


FIG.  55.  DIAGRAMS  OF  SEX  DETERMINATION  IN  THE  THREAD  WORM, 
Ascaris.  The  X  chromosomes  (black)  are  here  joined  to  ordinary  chro- 
mosomes, there  being  two  in  the  egg  mother  cell  and  one  in  the  sperm 
mother  cell.  All  eggs  contain  one  of  these  X  chromosomes,  while  half  of 
the  spermatozoa  have  it  and  half  do  not.  Eggs  fertilized  by  one  type  of 
sperm  produce  females,  those  fertilized  by  the  other  type  produce  males. 
(From  Wilson.) 


1 62  Heredity  and  Environment 

thumb,  from  one  hand  while  all  females  have  the  full  number 
on  both  hands.  When  the  hands  (gametes)  are  struck  together 
as  in  the  game  of  "bean  porridge"  there  will  be  an  equal  number 
of  cases  in  which  a  hand  with  five  digits  meets  one  with  five 
(female)  and  one  with  four  meets  one  with  five  (male).  In  the 
latter  case  there  will  be  an  "odd"  thumb  (chromosome)  which 
has  no  mate. 

Sex-  Determination  in  Man. — Even  in  man  sex  is  determined  in 
the  same  manner,  according  to  several  recent  investigators.  Wini- 
warter  concluded  that  there  are  in  the  spermatogonia  of  man  47 
chromosomes,  one  of  which  is  the  X  or  accessory  chromosome. 
These  unite  in  synapsis  into  23  pairs,  leaving  the  X  chromosome 
unpaired;  in  the  reduction  division  the  pairs  separate,  while  the 
X  chromosome  goes  entire  into  one  of  the  daughter  cells,  which 
consequently  contains  23  -{-  X  chromosomes,  whereas  the  other 
daughter  cell  contains  23  chromosomes.  In  the  female  there  are 
probably  48  chromosomes,  there  being  two  X  chromosomes,  one 
from  each  parent,  and  after  the  reduction  divisions  every  egg 
contains  24  chromosomes.  Winiwarter  held  that  if  an  egg  is 
fertilized  by  a  sperm  containing  24  chromosomes  an  individual 
with  48  chromosomes,  or  a  female,  is  produced;  if  fertilized  by  a 
sperm  with  23  chromosomes  an  individual  with  47  chromosomes, 
or  a  male,  results. 

It  is  a  curious  fact  that  it  has  been  very  difficult  to  determine 
the  exact  number  of  chromosomes  in  man.  This  is  probably  due 
to  the  difficulty  of  preserving  in  an  unaltered  condition  the  chro- 
mosomes of  mammals  in  general,  as  McClung  and  his  pupils  have 
shown,  and  also  to  the  peculiar  difficulty  of  obtaining  human  tis- 
sues in  a  perfectly  fresh  and  normal  condition.  Thus  Guyer  and 
Montgomery  found  not  47  but  about  22  chromosomes  in  the 
spermatogonia  of  man.  Since  both  the  latter  investigators 
worked  on  negroes  whereas  Winiwarter  worked  on  white  men  it 
was  suggested  by  Morgan  and  Guyer  that  there  may  be  twice 
as  many  chromosomes  in  the  white  race  as  in  the  black.  A 


The  Cellular  Basis 


similar  condition  in  which  one  race  has  twice  as  many  chro- 
mosomes as  another  race  of  the  same  species  is  found  in  two 
races  of  the  thread  worm,  Ascaris  megalocephala.  However,  more 
recent  work  has  shown  that  the  number  of  chromosmes  in  white 
men  and  in  negroes  is  the  same.  Wieman  found  24  chromosomes 
in  the  spermatogonia  (  ?)  of  both  races  and  he  inferred  that  in 
the  male  there  is  an  XY  pair  of  sex  chromosomes,  in  the  female  an 
XX  pair.  On  the  other  hand,  Evans*  found  in  hundreds  of  counts 
that  there  were  constantly  48  chromosomes  in  the  spermatogonia 
of  a  white  man,  thus  indicating  the  presence  of  an  XY  pair  of 
chromosomes  in  the  male  and  an  XX  pair  in  the  female. 

The  most  recent  work  on  the  number  of  chromosomes  in  man 
is  by  Painter,*  who  has  studied  normal  testes  of  both  whites  and 


Mr* 


FIG.  56.  SPERMATOGNIAL  CHROMOSOMES  OF  NE*GRO.  A.  Spermatogonium 
showing  48  chromosomes,  the  smallest  of  which  is  the  sex  chromosome  Y; 
the  other  sex  chromosome  X,  is  a  small  rod-shaped  one.  As  reproduced 
the  figure  is  magnified  about  3000  diameters.  From  a  preparation  and 
drawing  by  Painter. 

B.  The  same  chromosomes  spread  out  and  arranged,  according  to  size 
and  shape,  in  24  synaptic  pairs. 


*  See  Babcock  and  Clausen,  p.  538. 


164  Heredity  and  Environment 

negroes,  removed  in  castration  and  fixed  immediately  by  the  most 
approved  methods.  He  finds  that  in  both  whites  and  blacks,  there 
are  48  chromosomes  in  the  spermatogonia,  or  24  synaptic  pairs  in 
the  first  maturation  division;  one  of  these  is  plainly  the  XY 
pair,  the  X  and  Y  being  unequal  in  size.  I  am  indebted  to  Painter 
for  the  privilege  of  using  one  of  his  unpublished  drawings  (Fig. 
56A)  and  for  permission  to  quote  his  conclusions.  Guyer  has  also 
informed  me  personally  that  in  new  and  better  material  he  now 
finds  48  chromosomes  in  human  spermaftogonia,  one  of  these  being 
an  X  and  another  a  Y.  These  discoveries  appear  to  settle  once 
for  all  this  vexing  question  and  to  establish  the  fact  that  in  man,  as 
well  as  in  many  other  animals,  sex  is  determined  by  the  chromo- 
somes, the  sex  chromosomes  being  XX  in  the  female,  and  XY 
in  the  male. 

Sex  a  Mendelian  Character — Correlations  Between  chromo- 
somes and  sex  have  been  observed  in  more  than  one  hundred 
species  of  animals  belonging  to  widely  different  phyla.  In  a  few 
classes  of  animals,  particularly  Lepidoptera  and  birds,  the  evi- 
dence while  not  entirely  convincing  seems  to  point  to  the  fact 
that  two  types  of  ova  are  produced  and  but  one  type  of  sperma- 
tozoa; but  the  general  principle  that  sex  is  determined  by  the 
chance  union  of  male-producing  or  female-producing  gametes  is 
not  changed  by  such  cases. 

Sex,  therefore,  appears  to  be  inherited,  that  is,  its  factors  are 
present  in  the  germ  cells  but  probably  not  as  particular  genes  oc- 
cupying definite  loci  in  a  chromosome  but  rather  as  a  relation  of 
whole  chromosomes,  such  as  XX,  XY  or  XO  (see  p.  172)  ;  it  is  a 
Mendelian  character  in  which  the  female  is  usually  homozygous 
for  sex  while  the  male  is  heterozygous.  Consequently  in  the 
formation  of  the  gametes  every  egg  cell  receives  one  sex  de- 
terminer, while  only  one-half  of  the  spermatozoa  receive  such  a 
determiner,  the  other  half  of  them  being  without  it.  If  then  an 

*  Painter,  T.  S. — The  Y  Chromosome  in  Mammals,  Science,  May  27,  1921. 
Also  letter  dated  November  3,  1921. 


The  Cellular  Basis 


egg  is  fertilized  by  a  sperm  with  one  of  these  determiners  a  fe- 
male is  produced,  if  by  a  sperm  without  the  sex  determiner  a  male 
results.  This  is  graphically  illustrated  in  Fig.  57  in  which  X 
represents  the  sex-determiner,  which  is  duplex  in  the  female  and 
simplex  in  the  male,  and  the  chance  unions  of  male  and  female 
germ  cells  yields  females  (XX)  and  males  (XO)  in  equal  num- 
bers. Of  course  there  is  no  such  thing  as  a  "sex-producing" 
chromosome,  sex  being  a  developed  character  which  is  the  result 
of  many  intrinsic  and  extrinsic  causes.  The  A"  chromosome  is 
only  one  factor  in  the  development  of  sex  but  if  it  is  a  factor 
which  differs  in  the  case  of  the  two  sexes,  it  is  a  "sex-differential." 
2.  Environmental  Influence.  Alteration  of  Sex  Ratios. — On 
the  other  hand  there  are  many  observations  which  seem  to  indi- 


Garnetes 


FIG.  57.  DIAGRAM  SHOWING  SEX  AS  A  MENDELIAN  CHARACTER,  the  female 
being  homozygous,  the  male  heterozygous  for  sex.  The  female  forms 
gametes  all  of  which  contain  the  X  chromosome ;  the  male  forms  two  sorts 
of  gametes  one-half  of  which  contain  the  X  chromosome  and  the  other 
half  lack  it.  All  possible  combinations  of  these  gametes  give  2:2  or  i:  I 
ratio  of  females  to  males. 


166  Heredity  and  Environment 

cate  that  the  sex  ratio  may  be  changed  by  environmental  condi- 
tions acting  before  or  after ,  fertilization  and  therefore  it  has 
been  concluded  that  sex  is  determined  by  extrinsic  rather  than  by 
intrinsic  causes.  Many  of  these  observations,  as  already  remarked, 
are  now  known  to  be  erroneous  or  misleading,  since  they  do  not 
prove  what  they  were  once  supposed  to  demonstrate.  But  there 
remain  a  few  cases  which  cannot  at  present  be  explained  away  in 
this  manner.  Perhaps  the  best  attested  of  these  are  the  observa- 
tions of  R.  Hertwig  and  some  of  his  pupils  on  the  effect  of  the  time 
of  fertilization  on  the  determination  of  sex.  If  frog's  eggs,  which 
are  always  fertilized  after  they  are  laid,  are  kept  for  some  hours 
before  spermatozoa  are  mixed  with  them,  or  if  the  female  is  pre- 
vented for  two  or  three  days  from  laying  the  eggs  after  they  have 
entered  the  oviducts,  the  proportion  of  males  to  females  is  enor- 
mously increased.  A  wholly  similar  result  has  been  observed  by 
Pearl  and  Parshley  in  the  case  of  cattle,  where  delayed  fertiliza- 
tion of  the  egg  leads  to  a  great  preponderance  of  males.  Hertwig 
attempts  to  explain  his  extremely  interesting  and  important  ob- 
servations as  due  to  the  relative  size  of  nucleus  and  cytoplasm  of 
the  egg ;  but  in  general  this  nucleus-plasma  ratio  may  vary  greatly 
irrespective  of  sex  and  there  is  no  clear  evidence  that  it  is  a  cause 
of  sex  determination. 

Miss  King  also,  working  on  toad's  eggs,  has  increased  the 
proportion  of  females  by  slightly  drying  the  eggs  or  by  with- 
drawing water  from  them  by  placing  them  in  solutions  of  salts, 
acids,  sugar,  etc.,  but  the  manner  in  which  drying  increases  the 
proportion  of  females  is  wholly  unknown.  All  of  these  experi- 
ments on  sex  determination  in  frogs  and  toads  are  somewhat 
complicated  by  the  difficulty  of  determining  the  sex  of  tadpoles  and 
young  animals  and  the  evidence  is  by  no  means  conclusive  that  in 
these  cases  sex  is  determined  by  extrinsic  causes. 

Quite  recently  Whitney-  has  shown  that  in  several  species  of 
rotifers  a  scanty  diet  produces  in  the  second  filial  generation  only 
female  offspring  while  a  copious  diet  produces  as  high  as  95  per 


The  Cellular  Basis  167 

cent  of  male  offspring.  Many  earlier  investigators  had  found  that 
food  influences  sex,  though  usually  it  was  held  that  scanty  food  led 
to  the  production  of  males  and  abundant  food  to  females,  but  this 
older  work,  unlike  Whitney's,  was  generally  uncritical.  A.  F. 
Shull  finds  that  "the  irrevocable  event  leading  to  the  determination 
of  the  sex  of  any  given  parthenogenetically  produced  individual 
(rotifer)  occurs  in  the  maturation  of  the  egg  from  which  that  in- 
dividual's mother  develops  ....  Probably  a  definite  chemical 
change  in  the  proteins  of  the  chromosomes  occurs  at  the  time  of 
maturation."  The  diet  may  thus  affect  the  chromosomes  and 
through  these  the  sex. 

Extensive  statistics  show  that  in  many  animals  including  man 
more  males  are  born  than  females,  whereas  according  to  the  chro- 
mosome theory  of  sex-determination  as  many  female-producing 
spermatozoa  are  formed  as  male-producing  ones.  It  is  possible 
to  explain  such  departure  from  the  I :  i  ratio  of  males  to  fe- 
males in  conformity  with  the  chromosome  theory  if  one  class  of 
spermatozoa  are  more  active  or  have  greater  vitality  than  the 
other  class,  or  if  after  fertilization  one  sex  is  more  likely  to  live 
than  the  other.  In  the  human  species  it  is  known  that  mortality 
is  greater  in  male  babies  before  and  after  birth  than  in  female 
babies,  but  if  before  fertilization  the  activity  or  vitality  of  male- 
producing  spermatozoa  is  greater  than  that  of  female-producing 
ones  it  would  offer  a  possible  explanation  of  the  greater  number  of 
males  than  of  females  at  the  time  of  birth.  In  certain  insects  it  is 
known  that  only  females  develop  from  fertilized  eggs,  and  in 
one  of  these  cases,  viz.,  Phylloxera,  Morgan  has  discovered  that 
this  is  due  to  the  fact  that  all  the  male-producing  spermatozoa 
degenerate  and  that  only  female-producing  spermatozoa  become 
functional.  Possibly  experimental  alterations  of  the  sex  ratio, 
such  as  Hertwig,  King,  Whitney  and  others  have  brought  about 
may  be  explained  as  due  to  a  differential  action  of  the  modified 
egg  cells  or  of  the  environment  upon  the  two  types  of  sperma- 
tozoa. In  the  Drosophila  work  many  lethal  mutations  have  ap- 


1 68  Heredity  and  Environment 

peared  which  cause  the  early  death  of  those  zygotes  in  which  the 
lethal  gene  is  not  balanced  by  a  normal  allelomorph.  Some  of 
these  lethals  are  sex-linked  and  alterations  of  the  normal  sex  ratio 
in  certain  cases  may  be  explained  as  the  result  of  these  lethal 
factors.  Finally  the  chromosomal  theory  of  sex  determination  is 
so  well  supported  in  so  many  instances  where  at  first  it  seemed 
impossible  of  application,  that  it  ought  not  to  be  rejected  until 
unmistakable  evidences  can  be  adduced  against  it. 

3.  Hermaphrodites  and  Intersexes. — Finally  a  number  of  cases 
have  been  brought  to  light  which  indicate  the  necessity  of  dis- 
tinguishing between  the  hereditary  determination  of  sex  and  its 
ontogenetic  development.  It  has  been  known  for  a  long  time 
that  in  bees  and  ants  the  workers  are  imperfect  females,  while 
the  queens  are  perfect  females,  and  that  the  kind  or  amount  of 
food  which  is  fed  to  the  larvae  determines  whether  they  will  be 
workers  or  queens  (see  p.  233).  Again  in  many  animals  the 
development  of  male  or  female  characters  is  dependent  upon 
internal  secretions  or  hormones  from  the  sex  glands  or  other 
organs  (see  pp.  234-237).  In  these  cases  it  is  evident  that  sex 
was  determined  at  an  early  stage,  probably  at  fertilization,  but 
the  development  of  these  male  or  female  characters,  which  occurs 
later,  is  influenced  also  by  the  external  or  internal  environment  (p. 
218).  Breeders  of  cattle  are  familiar  with  the  fact  that  when 
twin  calves  are  of  opposite  sex,  the  male  is  sexually  perfect,  but 
the  female  usually  has  many  male  characters  and  grows  into  a 
steer-like  animal  which  is  sterile  and  is  known  as  a  "free  martin." 
In  a  recent  paper  Lillie  has  shown  that  in  all  such  cases  the  twins 
are  connected  by  blood  vessels  at  an  early  stage  in  utero  and  that 
there  is  a  more  or  less  complete  circulation  of  blood  from  one 
foetus  to  the  other,  and  he  concludes  that  "sex  hormones,"  which 
are  probably  formed  earlier  in  the  male  than  in  the  female,  are 
carried  from  the  male  to  the  female  twin,  thus  causing  the  de- 
velopment of  male  organs  in  an  animal  which  would  otherwise 
have  been  a  female.  Therefore  the  chromosomal  or  "zygotic 


The  Cellular  Basis  169 

determination  of  sex  is  not  irreversible  predestination  but  a  quan- 
titative overbalance  in  the  direction  of  one  sex  or  the  other" 
which  may  later  be  changed.  Somewhat  similar  conclusions  had 
previously  been  reached  by  Whitman  and  by  Riddle  regarding  the 
sex  of  pigeons,  by  Shull  in  the  case  of  Lychnis,  and  especially  by 
Goldschmidt  for  the  gypsy-moth.  Goldschmidt  supposes  that 
sex  is  determined  by  certain  enzymes  which  he  calls  "andrase" 
and  "gynase" ;  an  excess  of  the  former  leads  to  the  development 
of  males,  an  excess  of  the  latter  to  females,  and  varying  mixtures 
of  the  two  to  varying  intergrades  or  "intersexes."  He  assumes 
that  these  enzymes  are  present  in  the  "sex  determining"  chro- 
mosomes at  fertilization  as  well  as  in  later  stages  and  thus  he 
attempts  to  identify  all  sex  determining  factors  with  these  "sex 
enzymes."  The  difference  between  determination  by  chromo- 
somes and  by  internal  secretions,  that  is,  between  heredity  and 
development,  is  found  chiefly  in  the  time  at  which  these 
enzymes  act. 

Morgan  has  shown  that  "gynandromorphs"  or  "sex  mosaics" 
are  due  to  the  irregular  distribution  or  loss  of  certain  "sex 
chromosomes"  owing  to  abnormalities  in  fertilization  or  cleavage. 
In  such  cases  one  portion  of  the  body  shows  male  characters,  an- 
other portion  female  ones  and  a  study  of  the  chromosomes  in 
these  regions  shows  that  in  the  former  the  male  combination  of 
chromosomes  is  present,  in  the  latter  the  female  combination. 
This  comes  as  near  to  a  demonstration  of  the  truth  of  the  chro- 
mosomal theory  of  sex  determination  as  is  possible. 

Finally,  the  most  notable  recent  work  on  this  subject  is  by 
Bridges.*  In  his  studies  of  the  pomace  fly,  Drosophila  melano- 
gaster,  he  found  intersexes  whose  genetical  behavior  was  such  as 
to  suggest  that  they  had  more  or  less  than  the  usual  number  of 
chromosomes.  By  means  of  breeding  experiments  as  well  as  by 
microscopical  study  O'f  their  germ  cells  he  has  demonstrated  that 

*Bridges,  C.  B. — Triploid  Intersexes  in  Drosophila  Melanog  aster. 
Science,  Sept  16,  1921. 


170  Heredity  and  Environment 

this  is  true.  In  the  normal  fly  of  this  species  there  are  four  pairs 
of  chromosomes  (Fig.  65)  ;  the  first  pair  (Chromosomes  I)  are 
the  sex  chromosomes  which  are  XX  in  the  female  and  XY  in  the 
male;  the  second  and  third  pairs  (Chromosomes  II  and  III)  are 
large  and  V-shaped;  the  fourth  pair  (Chromosomes  IV)  are  very 
small  and  round.  Through  the  failure  of  chromosome  pairs  to 
separate  in  the  maturation  divisions  an  egg  or  sperm  may  come 
to  contain  an  abnormal  number  of  any  or  all  of  these  chromo- 
somes. 

As  opposed  to  the  sex  chromosomes,  all  others  are  known  col- 
lectively as  "autosomes."  A  normal  female  has  two  X  chromo- 
somes and  two  of  each  of  the  autosomes ;  a  normal  male  has  one 
X  and  two  of  each  of  the  others ;  but  when  two  X's  occur  with 
three  of  each  of  the  others,  or  with  three  of  some  of  them,  inter- 
sexes  result,  and  these  may  grade  all  the  way  from  perfect  females 
to  perfect  males  depending  upon  the  ratio  of  X  chromosomes  to 
autosomes.  Sex  is  therefore  determined  by  a  quantitative  rela- 
tion of  the  X  chromosomes  to  the  autosomes,  and  if  one  assumes 
that  there  are  sex  enzymes,  such  as  Goldschmidt  postulated,  it 
is  probable  that  the  X  chromosomes  produce  mainly  "gynase" 
and  the  autosomes  "andrase."  Such  an  explanation  harmonizes 
well  not  only  with  all  that  is  known  regarding  the  chromosomal 
determination  of  sex  and  of  intersexes  but  also  with  much  that 
is  known  concerning  the  possibility  of  modifying  the  development 
of  sex  by  enzymes  or  hormones  from  glands  of  internal  secretion. 

In  either  sex  many  secondary  sexual  characters  of  the  other 
sex  are  present  during  development  and  traces  of  these  may  per- 
sist in  the  adult;  but  one  set  of  these  characters  develops  fully 
in  the  male  and  another  set  in  the  female,  so  that  they  may  be 
called  sex-limited.  The  development  of  the  secondary  sex  char- 
acters is  usually  determined  by  internal  secretions  from  the  ova- 
ries or  testes,  though  in  some  cases  they  may  develop  after  these 
organs  have  been  removed,  but  in  the  last  analysis  both  primary 
and  secondary  sex  characters  are  dependent  upon  the  sex  deter- 


The  Cellular  Basis  171 

miners  in  the  germ  cells.  Sex  and  sex-limited  inheritance  are 
only  special  cases  of  Mendelian  inheritance  and  the  full  develop- 
ment of  the  male  or  female  condition  is  dependent  upon  the 
predominance  of  male-determining,  or  of  female-determining 
factors,  both  hereditary  and  environmental;  while  the  condition 
of  "intersexes"  is  the  result  of  the  lack  of  such  predominance. 

On  one  point  there  is  general  agreement,  namely  every  organ- 
ism is  at  the  beginning  of  ontogeny  so  evenly  balanced  between 
maleness  and  femaleness  that  very  slight  changes  in  heredity  or 
environment  may  cause  it  to  go  one  way  or  the  other ;  every  or- 
ganism is  potentially  both  male  and  female,  and  even  in  the  fully 
developed  state  each  sex  carries  the  vestiges  of  suppressed  organs 
of  the  other  sex. 

D.    THE  MECHANISM  OF  HEREDITY 

The  mechanism  of  heredity,  as  contrasted  with  the  mechanism 
of  development,  consists  in  the  formation  of  particular  kinds  of 
germ  cells  and  in  the  union  of  certain  of  these  cells  in  fertiliza- 
tion. We  have  briefly  traced  the  origin,  maturation  and  union  of 
male  and  female  sex  cells  in  a  number  of  animals,  and  in  these 
phenomena  we  have  the  mechanism  of  the  hereditary  continuity 
between  successive  generations.  But  in  addition  to  these  specific 
facts  there  are  certain  general  considerations  which  need  to  be 
emphasized. 

I.    THE  SPECIFICITY  OF  GERM  CELLS 

The  conclusion  is  inevitable  that  the  germ  cells  of  different 
species  and  even  those  of  different  individuals  are  not  all  alike. 
Every  individual  difference  between  organisms  must  be  due  to 
one  or  more  differentiating  causes  or  factors.  Specific  results 
come  only  from  specific  causes.  These  causes  may  be  found  in  the 
organization  of  the  germ  cells  or  in  environmental  stimuli,  i.e., 
they  may  be  intrinsic  or  extrinsic,  but  as  a  matter  of  fact  experi- 
ence has  shown  that  they  are  generally  intrinsic  in  the  germ.  In 


172  Heredity  and  Environment 

the  same  environment  one  egg  becomes  a  chicken  and  another  a 
duck;  one  becomes  a  frog  and  another  a  fish  and  another  a 
snail;  one  becomes  a  black  guinea-pig  and  another  a  white  one; 
one  becomes  a  male  and  another  a  female ;  one  gives  rise  to  a  tall 
man  and  another  to  a  short  man,  etc.  Since  these  differences  may 
occur  in  the  same  environment  they  must  be  due  to  differences  in 
the  germ  cells  concerned. 

Environment  Non-specific. — On  the  other  hand  different  en- 
vironmental conditions  may  be  associated  with  similar  develop- 
mental results.  Loeb  and  others  have  found  that  artificial  par- 
thenogenesis may  be  induced  by  a  great  variety  of  environmental 
stimuli,  viz.,  by  salt  solutions,  by  acids  and  alkalis,  by  fatty  acids 
and  fat  solvents,  by  alkaloids  and  cyanides,  by  blood  serum  and 
sperm  extract,  by  heat  and  cold,  by  agitation  and  electric  current. 
There  is  certainly  nothing  specific  in  these  different  stimuli. 
Similarly  Stockard  has  discovered  that  cyclopia,  or  one-eyed 
monsters,  may  be  produced  by  magnesium  salts,  alcohol,  chlore- 
tone,  chloroform,  and  ether,  and  to  this  list  McClendon  has 
added  various  other  salts  and  anaesthetics.  In  all  such  cases  it  is 
evident  that  the  specific  results  of  such  treatment  are  due  to  a 
specific  organization  of  the  germ  rather  than  to  specific  stimuli, 

Why  does  one  egg  give  rise  to  a  chicken  and  another  to  a  duck, 
or  a  fish,  or  a  frog?  Why  does  one  egg  give  rise  to  a  black  guinea- 
pig  and  another  to  a  white  one,  though  both  may  be  produced  by 
the  same  parents  ?  Why  does  one  child  differ  from  another  in  the 
same  family  ?  Why  does  one  cell  give  -rise  to  a  gland  and  another 
to  a  nerve,  one  to  an  egg  and  another  to  a  sperm?  If  these  dif- 
ferences are  not  due  to  environmental  causes,  and  the  evidence 
shows  that  they  are  not,  they  must  be  due  to  differences  in  the 
structures  and  functions  of  the  cells  concerned. 

Protoplasm  Specific. — Many  differences  in  the  material  sub- 
stances of  cells  are  visible,  and  many  more  are  invisible  though 
still  demonstrable.  These  differences  may  not  be  detectable  by 
chemical  or  physical  tests,  and  yet  they  may  be  demonstrated 


The  Cellular  Basis  173 

i 
physiologically  and  developmentally.     The  most  delicate  of  all 

tests  are  physiological,  as  is  shown  by  the  Weidal  test  in  typhoid 
fever,  the  Wassermann  reaction  in  syphilis,  the  reactions  of  im- 
munized animals  to  different  toxins,  etc.  Lillie  has  recently 
shown  that  egg  cells  give  off  a  substance  which  he  calls  "fertil- 
izing which  can  be  detected  only  by  the  way  in  which  spermato- 
zoa react  to  it.  No  chemical  or  physical  test  can  distinguish  be- 
tween the  different  eggs  or  spermatozoa  produced  by  the  same 
individual,  but  the  reactions  of  these  cells  in  development  prove 
that  they  are  different.  Undoubtedly  chemical  and  physical  dif- 
ferences are  here  present  but  no  chemical  methods  at  present 
available  are  sufficiently  delicate  to  detect  them. 

It  is  one  of  the  marvelous  facts  of  biology  that  practically 
every  sexually  produced  individual  is  unique,  the  first  and  last 
of  its  identical  kind,  and  although  some  of  these  individual  dif- 
ferences are  due  to  varying  environment,  others  are  evidently  due 
to  germinal  differences,  so  that  we  must  conclude  that  every 
fertilized  egg  cell  differs  in  some  respects  from  every  other  one. 

But  are  there  molecules  and  atoms  enough  in  a  tiny  germ  cell, 
such  as  a  spermatozoon,  to  allow  for  all  these  differences?  Mie- 
scher  has  shown  that  a  molecule  of  albumin  with  40  carbon  atoms 
may  have  as  many  as  one  billion  stereo-isomers,  and  in  protoplasm 
there  are  many  kinds  of  albumin  and  other  proteins,  some  with 
probably  more  than  700  carbon  atoms-  In  such  a  complex  sub- 
stance as  protoplasm  the  possible  variations  in  molecular  con- 
stitution must  be  well  nigh  infinite,  and  it  can  not  be  objected  on 
this  ground  that  it  is  chemically  and  physically  impossible  to 
have  as  many  varieties  of  germ  cells  as  there  are  different  kinds 
of  individuals  in  the  world. 

Permutation's  of  Chromosomes. — Even  with  regard  to  morpho- 
logical elements  which  may  be  seen  with  the  microscope  it  can  be 
shown  that  an  enormous  number  of  permutations  is  possible  (Fig. 
58).  It  seems  probable,  as  Boveri  has  shown,  that  different 
chromosomes  of  the  fertilized  egg  differ  in  hereditary  potencies, 


174  Heredity  and  Environment 

and  where  the  number  of  chromosomes  is  fairly  large  the  number 
of  possible  combinations  of  these  chromosomes  in  the  germ  cells 
becomes  very  great.  In  man,  where  there  are  probably  48  chro- 
mosomes and,  after  synapsis,  24  pairs  of  maternal  and  paternal 
ones,  the  possible  number  of  permutations  in  the  distribution 
of  these  chromosomes  to  the  different  germ  cells  would  be  224, 
or  16,777,216,  and  the  possible  number  of  different  combinations 
of  fertilized  eggs  or  oosperms  which  could  be  produced  by  a  single 
pair  of  parents  would  be  (16,777,21 6) 2,  or  approximately  three 
hundred  thousand  billions.f  But  probably  other  things  than  chro- 
mosomes differ  in  different  germ  cells,  and  it  is  by  no  means  cer- 
tain that  individual  chromosomes  are  always  composed  of  the  same 
chromomeres,  or  units  of  the  next  smaller  order,  and  in  view  of 
these  possibilities  it  may  well  be  that  every  human  germ  cell 
differs  morphologically  and  physiologically  from  every  other 
one,  in  short  that  every  oosperm  and  every  individual  which  de- 
velops from  it  is  absolutely  unique. 

Significance  of  Sexual  Reproduction. — Indeed  the  production 
of  unique  individuals  seems  to  be  the  chief  purpose  and  result  of 
sexual  reproduction.  In  asexual  reproduction  the  individual  var- 
iations which  occur  are  chiefly  if  not  entirely  due  to  environment, 
but  in  sexual  reproduction  they  are  also  due  to  new  combina- 
tions of  hereditary  elements.  The  particular  germinal  organiza- 
tion transmitted  from  one  generation  to  the  next  depends  upon 
(a)  the  organization  inherited  from  ancestors,  (b)  the  particular 
character  of  the  cell  divisions  by  which  the  germ  cells  are  formed, 
(c)  the  particular  kinds  of  egg  and  sperm  cells  which  combine  in 
fertilization.  The  inherited  organization  determines  all  the  gen- 
eral characteristics  of  race,  species,  genus,  order,  phylum.  It  de- 
termines the  possibilities  and  limitations  of  individual  variations. 
Given  a  certain  inherited  organization,  the  individual  peculiarities 

t  Excluding  duplications  there  would  be  324  different  genotypes  and  224 
different  phenotypes,  assuming  that  chromosomes  always  preserve  their 
identity  and  that  dominance  is  always  complete. 


The  Cellular  Basis 


175 


1 9  GAMETES    |      ZYGOTES     |    6  GAMETES  I 


FIG.  58.  DIAGRAM  SHOWING  SOME  OF  THE  POSSIBLE  DISTRIBUTIONS  OF 
CHROMOSOMES  OF  GRANDPARENTS  TO  GRANDCHILDREN.  Chromosomes  of 
maternal  grandfather  shaded  black,  of  maternal  grandmother  with  'cross 
lines;  of  paternal  grandmother  stippled,  of  paternal  grandfather  un- 
shaded. "Sex-chromosomes"  are  /-shaped,  a  pair  being  present  in  the 
female,  a  single  one  in  the  male.  In  the  maturation  of  oocyte  and  of  sper- 
matocyte  homologous  chromosomes  unite  in  pairs  and  then  separate  into 
the  gametes.  With  6  pairs  of  chromosomes,  each  free  to  separate  in  2 
directions  the  largest  possible  number  of  combinations  in  the  gametes  is 
(2)6  or  64  and  since  any  sperm  may  unite  with  any  egg,  the  number  of 
combinations  possible  in  the  zygotes  is  (64) 2  or  4096  and,  excluding  all 
duplications,  there  are  (3)6  or  729  genotypes  only  4  of  which  are  shown 
in  the  diagram. 


176  Heredity  and  Environment 

of  the  germ  cells  are  determined  by  the  particular  character  of 
cell  division  by  which  the  germ  cells  are  formed,  and  the  peculiari- 
ties of  the  individuals  or  persons  which  develop  from  these  cells 
are  determined  in  large  part  by  the  particular  kinds  of  germ  cells 
which  unite  in  fertilization. 

Comparison  of  Cards  and  Chromosomes. — The  behavior  of 
chromosomes  in  maturation  and  fertilization  is  like  the  shuffle 
and  deal  of  cards  in  a  game,  and  apparently  with  the  same  object, 
namely,  never  to  deal  the  same  hand  twice.  To  make  this  compari- 
son more  complete  suppose  that  kings  be  discarded  from  the 
pack,  leaving  48  cards  of  two  colors,  red  and  black,  which  we  will 
compare  to  the  48  chromosomes  of  maternal  and  paternal  origin ; 
suppose  that  in  the  shuffling  of  these  cards  corresponding  cards  of 
the  red  and  black  suits  are  temporarily  stuck  together  so  that  the 
ace  of  diamonds  is  united  with  the  ace  of  clubs,  the  queen  of 
hearts  with  the  queen  of  spades,  etc.,  thus  forming  24  red-black 
pairs  of  the  same  denominations.  If  these  cards  are  then  dealt 
into  two  hands,  one  card  of  each  pair  going  to  one  hand  and  the 
other  to  the  other  hand,  we  will  have  two  cards  of  each  denomina- 
tion in  each  hand,  but  if  the  cards  are  dealt  indiscriminately  each 
hand  may  contain  two  black  cards  of  any  denomination,  two  red 
cards  or  one  black  and  one  red-  This  description  parallels  what 
takes  place  in  the  maturation  of  the  human  germ  cells,  except  that 
there  is  no  evidence  that  there  are  more  than  two  suits  of  chromo- 
somes, one  of  which  is  maternal  and  the  other  paternal. 

If  now  we  complete  this  comparison  by  extending  it  to  what 
takes  place  in  fertilization  we  must  take  one  hand  from  each  of 
these  deals  and  put  them  together  into  one  pack ;  this  pack  would 
contain  cards  of  every  denomination  from  ace  to  queen  but  there 
would  be  varying  numbers  of  red  and  black  cards  and  a  mixture 
of  cards  from  two  distinct  packs.  In  no  game  of  cards  are  half 
of  the  cards  taken  from  one  pack  and  half  from  another  at  every 
game,  but  this  is  just  what  happens  in  the  shuffle  and  deal  of  the 
chromosomes.  Because  of  the  mixture  of  chromosomes  from  dis- 


The  Cellular  Basis  177 

tinct  individuals  in  every  generation,  each  of  which  has  its  own 
peculiar  value,  the  game  of  heredity  becomes  vastly  more  complex 
than  any  game  of  cards. 

This  illustration  may  serve  to  make  plain  the  fact  that  in  the 
process  of  maturation  and  fertilization  there  is  this  shuffle  and  deal 
of  the  chromosomes,  with  the  result  that  every  oosperm  and  every 
individual  which  develops  from  it  is  different  from  every  other 
one. 

Germ  Cells  as  Specific  as  Persons. — This  conception  of  the  spe- 
cificity of  every  germ  cell,  as  well  as  of  every  developed  individ- 
ual, sets  the  whole  problem  of  heredity  and  development  in  a  clear 
light.  The  visible  peculiarities  of  an  adult  become  invisible  as 
development  is  traced  back  to  the  germ,  but  they  do  not  wholly 
cease  to  exist.  Similarly  the  multitudinous  complexities  of  an 
adult  fade  out  of  view  as  development  is  traced  to  its  earliest 
stages,  but  it  is  probable  that  they  are  not  wholly  lost.  In  short, 
the  specificity  of  the  germ  applies  not  merely  to  those  things  in 
which  it  differs  from  other  germs,  but  also  to  characters  in  which 
it  resembles  others;  in  short,  to  hereditary  resemblances  no  less 
than  to  hereditary  differences. 

The  mistake  of  the  doctrine  of  preformation  (see  p.  57)  was 
in  supposing  that  germinal  parts  were  of  the  same  kind  as  adult 
parts;  the  mistake  of  epigenesis  was  in  maintaining  the  lack  of 
specific  parts  in  the  germ.  The  development  of  every  animal  and 
plant  consists  in  the  transformation  of  the  specific  characters  of 
the  germ  into  those  of  the  adult.  From  beginning  to  end  de- 
velopment is  a  series  of  morphological  and  physiological  changes 
but  not  of  new  formations  or  creations  except  in  so  far  as  new 
structures  or  functions  appear  as  a  result  of  "creative  synthesis." 
It  is  only  the  incompleteness  of  our  knowledge  of  development 
which  allows  us  to  say  that  the  eye  or  ear  or  brain  begins  to 
form  in  this  or  that  stage.  They  become  visible  at  certain  stages, 
but  their  real  beginnings  are  indefinitely  remote. 


178  Heredity  and  Environment 

II.     CORRELATIONS  BETWEEN  GERMINAL  AND  SOMATIC 
ORGANIZATION 

All  the  world  knows  that  the  organization  of  the  germ  is  not 
the  same  as  that  of  the  developed  animal  which  comes  from  it,  and 
yet  the  specificity  of  the  germ  indicates  that  there  must  be  some 
correlation  between  the  germinal  and  the  developed  organization ; 
in  short,  there  is  not  identity  of  organization  but  correlation  of 
organization  between  the  germ  and  the  adult.  What  correlations 
are  known  to  exist  between  the  oosperm  and  the  developed  ani- 
mal? 

Inheritance  Factors  and  Developed  Characters. — We  have  con- 
sidered in  the  preceding  chapter  (pp.  100-105)  the  evidence  that 
there  are  specific  inheritance  factors  or  genes  which  are  correlated 
with  the  development  of  specific  adult  characters.  These  factors 
are  not  the  characters  in  miniature  nor  are  they  the  "representa- 
tives" or  "carriers"  of  characters,  but  they  are  the  differential 
causes  of  characters.  Every  inherited  character  must  have  a 
differential  cause  in  the  germ  cells,  but  this  cause  may  not  be  a 
peculiar  vital  corpuscle  nor  even  a  peculiar  atom  or  molecule  but 
it  must  at  least  be  some  particular  combination  of  atoms  or  mole- 
cules. It  is  practically  certain  that  this  differential  cause  of  each 
inherited  character  is  associated  with  some  material  substance 
which  occupies  some  place  in  the  germ  cells. 

Location  of  Inheritance  Factors. — If  it  is  asked  whether  there 
are  particular  structures  in  germ  cells  which  correspond  to  par- 
ticular inheritance  factors  it  must  be  admitted  that  we  have  no 
certain  knowledge  on  this  subject  and  that  opinions  differ  greatly 
with  respect  to  it.  On  the  one  hand  it  is  maintained  that  the 
entire  germ  is  concerned  in  the  development  of  every  character, 
and  on  the  other  hand  that  the  differential  cause  of  any  character 
may  be  located  in  some  differentiated  structure  or  function  of  the 
germ.  These  views  are  not  mutually  exclusive  and  it  may  well 
be  that  both  are  true.  We  know  that  germ  cells  are  composed  of 


The  Cellular  Basis  179 

many  parts  which  differ  from  one  another  in  both  structure  and 
function,  and  it  is  highly  probable  that  there  are  enough  of  these 
parts  to  provide  a  locus  for  every  inheritance  factor. 

There  was  a  time  when  the  cell  was  the  Ultima  Thule  of  biologi- 
cal analysis  and  when  the  contents  of  cells  were  supposed  to  be 
"perfectly  homogeneous,  diaphanous,  structureless  slime."  Then 
the  nucleus  was  discovered  within  the  cell,  then  the  chromosomes 
within  the  nucleus,  then  the  chromomeres  within  the  chromosomes, 
and  there  is  no  reason  to  suppose  that  organization  ceases  with  the 
powers  of  our  present  microscopes.  With  every  improvement  of 
the  microscope  and  of  microscopical  technique,  structures  have 
been  found  in  cells  which  were  undreamed  of  before,  and  it  is 
not  probable  that  the  end  has  been  reached  in  this  regard.  We 
know  that  cells  contain  nuclei  and  chromosomes  and  chromo 
meres,  centrosomes  and  plastosomes  and  microsomes,  and  we 
know  that  some  of  these  parts  differ  in  function  as  well  as  struc- 
ture. And  there  is  no  reason  to  doubt  that  if  we  had  sufficiently 
powerful  microscopes  we  should  find  still  smaller  and  smaller 
units  until  we  came  at  last  to  molecules  and  atoms. 

The  fact  that  inheritance  units  from  the  two  parents  unite  in 
fertilization  and  later  segregate  in  the  formation  of  gametes,  so 
that  the  latter  are  pure  with  respect  to  any  character,  is  a  familiar 
part  of  Mendelian  inheritance  (Fig.  59).  Even  if  these  units  be 
regarded  as  physiological  processes  they  must  be  associated  with 
particular  structures,  since  function  and  structure  are  inseparable 
in  life  processes.  What  are  the  units  in  terms  of  cell  structures 
and  where  are  they  located  in  the  cell? 

i.  CHROMOSOMAL  INHERITANCE  THEORY. — We  have  in  .the 
chromosomes,  as  Wilson  especially  has  emphasized,  an  apparatus 
which  fulfils  all  the  requirements  of  carriers  of  Mendelian  fac- 
tors (Fig.  60).  Both  factors  and  chromosomes  come  in  equal 
numbers  from  both  parents;  both  maternal  and  paternal  factors 
and  chromosomes  pair  in  the  zygote  and  separate  in  the  gamete 
as  shown  in  Figs.  59  and  60;  and  so  far  as  is  known  the  chro- 


i8o 


Heredity  and  Environment 


mosomes  are  the  only  portions  of  the  germ  cells  which  ful- 
fil these  conditions.  The  association,  segregation  and  distribu- 
tion of  Mendelian  factors  and  of  maternal  and  paternal  chromo- 
somes are  exactly  parallel  and  it  is  not  reasonable  to  suppose  that 
this  remarkable  coincidence  is  without  significance. 

Furthermore  there  is  much  additional  evidence  that  the  chro- 
mosomes are  important  factors  in  heredity  and  development: 
Boveri  has  studied  the  abnormal  distribution  of  chromosomes  to 
different  cleavage  cells  in  doubly  fertilized  sea  urchin  eggs  and  has 
found  evidence  that  the  hereditary  value  of  different  chromosomes 
is  different.  McClung,  Stevens,  Wilson  and  others  have  dis- 
covered that  the  determination  of  sex  is  associated  with  the  pres- 
ence or  absence  of  a  particular  chromosome,  the  X  or  Y  chromo- 
some, in  the  spermatozoon  which  fertilizes  the  egg.  If  an  egg 


FIG.  59.  DIAGRAM  SHOWING  UNION  OF  FACTORS  IN  FERTILIZATION  AND 
THEIR  SEGREGATION  IN  THE  FORMATION  OF  GERM  CELLS.  With  4  pairs  of  fac- 
tors (Aa,  Bb,  Cc,  Dd)  16  types  of  gametes  are  possible  as  shown  in  the 
two  series  of  small  circles  at  the  right.  (After  Wilson.) 


The  Cellular  Basis 


181 


is  fertilized  by  a  sperm  which  lacks  the  X  chromosome  a  male  is 
produced,  if  fertilized  by  the  other  type  a  female  results.  This 
correlation  between  the  presence  or  absence  of  a  whole  chromo- 
some and  of  a  developed  character  such  as  sex,  was  the  first  case 
of  the  kind  that  was  known  and  more  than  anything  else  it 
served  to  prove  that  the  chromosomes  contain  the  Mendelian 
factors.  The  absence  of  a  whole  chromosome  is  plainly  visible 
under  the  microscope,  whereas  the  absence  of  a  single  factor  or 
gene  from  a  chromosome  would  never  have  been  seen.  These 
fortunate  cases  in  which  the  male  lacks  a  whole  chromosome  seem 
almost  to  have  been  intended  to  give  ocular  proof  of  the  chromo- 
somal theory  of  heredity;  they  are  to  biology  what  the  rings  of 
Saturn  are  to  astronomy, — a  visible  confirmation  of  a  great 
theory. 


Bynaprii 


AbeD 
*•*««•••  Oenn    Cell. 

Dii-iiion  Simple*     Group, 


Somatic     Division* 
Duple*    Group* 

FIG.  60.  DIAGRAM  OF  GERM  CELLS  CORRESPONDING  TO  FIG.  59,  show- 
ing the  union  of  maternal  chromosomes  (ABCD)  and  paternal  ones 
(abed)  in  fertilization,  their  distribution  in  cleavage,  their  union  into  4 
pairs  (Aa,  Bb,  Cc,  Dd)  in  synapsis  and  the  separation  of  the  pairs  in  the 
reduction  division.  Only  2  of  the  16  possible  types  of  germ  cells  are  shown 
at  the  lower  right.  (After  Wilson.) 


182  Heredity  and  Environment 

We  have  in  these  facts  a  remarkable  correlation  between  the 
distribution  of  the  chromosomes  and  the  occurrence  of  certain 
characters  of  the  adult  animal.  The  association  of  maternal  and 
paternal  chromosomes  in  fertilization  and  their  segregation  in 
the  maturation  of  the  germ  cells  is  parallel  to  the  association  of 
Mendelian  characters  in  the  zygote  and  their  segregation  in  the 
gametes;  if  the  distribution  of  chromosomes  in  cleavage  is  ab- 
normal the  larva  shows  abnormal  characters  (Boveri) ;  sex  de- 
termination is  associated  with  the  distribution  of  a  particular 
chromosome  to  one-half  of  the  spermatozoa,  and  the  fertilization 
of  the  egg  by  one  or  the  other  type  of  spermatozoa.  (Wilson). 
There  are  many  parts  of  a  germ  cell,  all  of  which  may  be  con- 
cerned in  heredity  and  development,  but  the  chromosomes  appear 
to  be  the  seat  of  the  differential  factors  for  Mendelian  characters. 

On  the  other  hand  it  has  been  objected  by  certain  investigators, 
notably  Child,  Foot  and  Strobell,  et  al,  that  chromosomes  are  not 
the  causes  of  anything,  but  that  they  are  the  "results  of  dynamic 
processes,"  "the  expression  rather  than  the  cause  of  cell  activi- 
ties." This  objection  seems  to  confuse  the  idea  of  natural  cause 
with  that  of  final  cause.  Science  knows  nothing  of  the  latter; 
any  natural  cause  is  only  a  link  in  the  chain  of  cause  and  effect,  it 
is  itself  the  result  of  antecedent  causes  and  the  cause  of  subse- 
quent results.  Undoubtedly  the  chromosomes  are  the  results  of 
antecedent  processes,  and  yet  they  may  also  be  the  causes  of  sub- 
sequent results.  No  thoughtful  person  has  ever  maintained  that 
chromosomes  or  any  other  things  in  nature  are  autonomous,  abso- 
lute, uncaused  causes. 

Abnormal  Distribution  of  Chromosomes  and  Factors. — Experi- 
mental evidence  that  the  chromosomes  are  the  seat  of  inheritance 
factors  is  found  in  the  correlation  between  the  abnormal  distribu- 
tion of  chromosomes  and  the  development  of  abnormal  characters 
in  the  embryo  or  adult.  An  abnormal  distribution  of  chromo- 
somes to  the  cleavage  cells  may  be  caused  in  a  variety  of  ways  but 
one  of  the  least  injurious  methods  of  accomplishing  this  is  by 


The  Cellular  Basis 


183 


causing  two  spermatozoa  instead  of  one  to  enter  an  egg.  In 
such  doubly  fertilized  eggs  Boveri  discovered  that  different  cleav- 
age cells  receive  a  different  number  of  chromosomes  and  in  gen- 
eral those  cells  which  receive  the  largest  number  develop  most 
typically,  while  those  which  receive  a  small  number  develop  atypi- 
cally.  By  a  skillful  analysis  Boveri  proved  that  normal  develop- 


PRIMARY  BOM -DISJUNCT ION    IN  MALE 


PRIMARY  HOB-DISJUNCTION  JH  FEMALE 


.SYGOTES 


FIG.  60  a.  DIAGRAM  OF  NON-DISJUNCTION  OF  THE  SEX  CHROMOSOMES  in 
the  Maturation  of  the  Egg  and  Sperm  of  Drosophila  melanogaster,  with 
the  resulting  types  of  Zygotes  (According  to  Bridges).  Primary  non- 
disjunction  in  the  sperm  leads  to  the  production  of  XXY  females  and 
XO  males;  primary  non-disjunction  in  the  egg  leads  to  four  combina- 
tions, two  of  which  (XXX  and  YO)  are  non-viable;  secondary  non- 
disjunction  occurs  in  the  maturation  of  an  XXY  egg.  In  Drosophila  Y  is 
an  "empty  chromosome,"  i.e.,  it  appears  to  contain  no  genes,  yet  it  has  some 
influence  since  a  male  without  it  (XO)  is  sterile.  Zygotes  are  matro- 
clinous  or  patroclinous  depending  upon  whether  they  get  a  larger  number 
of  X  chromosomes  from  the  egg  or  from  the  sperm. 


184  Heredity  and  Environment 

ment  depends  not  so  much  upon  the  absolute  number  of  chromo- 
somes in  a  given  cell  as  upon  a  complete  set  of  all  the  different 
kinds  of  chromosomes,  and  when  a  complete  set  was  not  present 
certain  characters  failed  to  develop.  By  this  means  he  showed 
that  different  chromosomes  of  a  set  differ  in  hereditary  value,  as, 
for  example,  the  fingers  of  a  hand  differ  from  one  another,  and 
that  two  chromosomes  of  one  kind  could  not  make  up  for  the 
lack  of  one  of  another  kind,  any  more  than  two  thumbs  could 
make  up  for  the  loss  of  a  little  finger. 

A  still  more  detailed  correlation  between  the  presence  or  absence 
of  a  particular  chromosome  and  the  presence  or  absence  of  parti- 
cular characters  in  the  developed  organism  has  been  described 
by  Bridges  (1916).  In  his  study  of  the  pomace  fly  Drosophila 
melanogaster,  he  found  that  the  occasional  appearance  (i  in  1700) 
of  a  matroclinous  daughter  or  patroclinous  son  was  due  to  the  fact 
that  the  members  of  the  XX  pair  of  chromosomes  of  the  oocyte 
fail  to  separate  in  the  reduction  division  so  that  both  XX's  are 
included  in  the  egg  (Fig.  65,  C)  or  both  are  extruded  in  the  polar 
body,  the  eggs  being  accordingly  XX  or  O;  or  the  two  chromo- 
somes of  the  XY  pair  in  the  spermatocyte  fail  to  separate  in  the 
reduction  division  so  that  one  sperm  may  have  both  X  and  Y, 
and  another  lack  both  of  these  chromosomes  (Fig.  60  a).  This 
phenomenon  he  calls  "non-disjunction"  and  it  results  in  the  pro- 
duction of  matroclinous  daughters  or  patroclinous  sons,  and  in 
many  other  irregularities  of  inheritance  which  follow  precisely 
the  abnormal  distribution  of  these  chromosomes.  A  patroclinous 
son  is  the  result  of  the  fertilization  of  an  O  egg  by  an  X  sperm ; 
such  an  XO  son  is  sterile  whereas  the  normal  XY  son  is  not,  thus 
showing  that  the  chromosome  Y  has  some  function  though  in 
Drosophila  it  does  not  contain  any  of  the  genes ;  fertilization  of  an 
O  egg  by  a  Y  sperm  produces  a  combination  which  is  non-viable. 
Fertilization  of  an  XX  egg  by  a  Y  sperm  produces  a  matroclinous 
daughter  (XXY),  whereas  fertilization  of  an  XX  egg  by  an  X 
sperm  produces  a  combination  (XXX)  which  is  non- viable. 


The  Cellular  Basis  185 

These  relations  are  shown  schematically  in  the  accompanying 
diagrams  (Fig.  60  a). 

2.  LINKAGE  OF  CHARACTERS  AND  CHROMOSOMAL  LOCALIZATION. 
— Finally  the  study  of  characters  which  are  linked  together  in 
heredity,  joined  with  the  study  of  the  chromosomes  and  their  dis- 
tribution in  the  maturation  and  fertilization  of  the  germ  cells,  has 
not  only  confirmed  the  chromosomal  theory  of  heredity  but  has 
also  shown  that  certain  chromosomes  carry  the  genes  for  certain 
characters  and  has  even  indicated  the  relative  positions  of  dif- 
ferent genes  in  the  chromosomes. 

Thanks  to  the  work  of  Bateson,  Morgan,  and  many  others,  it 
is  now  known  that  many  characters  are  linked  together  in  inheri- 
tance. Darwin  had  long  ago  noted  that  male  albino  cats  with  blue 
eyes  are  usually  deaf  and  many  other  cases  of  the  association  of 
peculiar  characters  had  been  reported  by  earlier  observers.  In 
1906,  Bateson  and  Punnett  found  that  sweet  peas  with  purple 
flowers  usually  have  elongated  pollen  grains,  whereas  those  with 
red  flowers  have  round  pollen.  Since  1910  Morgan  and  his  pu- 
pils have  discovered  about  four  hundred  new  characters,  or 
mutations  in  the  pomace  fly,  Drosophila  melanogaster,  which  are 
usually  linked  together  in  four  groups. 

a.  Sex-linked  Inheritance. — The  first  cases  of  such  linkage 
studied  by  Morgan  were  in  characters  which  are  usually  asso- 
ciated with  one  or  the  other  sex,  but  which  may  have  nothing  to 
do  with  reproduction  and  may  affect  any  part  of  the  body.  Such 
characters  are  not  necessarily  limited  to  one  sex  or  the  other,  as 
are  many  primary  and  secondary  sexual  characters,  but  they  may 
appear  in  either  sex  though  they  are  usually  transmitted  from 
fathers  to  daughters,  or  from  mothers  to  sons  ("criss-cross"  in- 
heritance) in  exactly  the  way  in  which  the  sex  chromosomes  (X) 
are  transmitted.  Morgan  has  therefore  concluded  that  the  factors 
for  these  characters  are  carried  by  the  sex  chromosomes  and  has 
named  them  sex-linked  characters.  In  the  pomace  fly,  Drosophila, 
he  has  discovered  a  large  number  of  such  characters  which  are 


1 86 


Heredity  and  Environment 


FIG.  61.  SEX -LINKED  INHERITANCE  OF  WHITE  AND  RED  EYES  IN  Dro- 
sophila.  Parents,  red-eyed  female  and  white-eyed  male  (females  are 
larger  than  males;  F,,  red-eyed  males  andefemales;  F2,  red-eyed  females 
and  equal  numbers  of  red-eyed  and  white-eyed  males.  The  distribution 
of  sex  chromosomes  is  shown  through  the  middle  of  the  figure,  the  straight 
rods  being  X-chromosomes  and  the  hooked  rods  Y-chromosomes ;  the 
X-chromosomes  that  carry  the  factor  for  red  eye  are  black,  those  that 
carry  the  factor  for  white  eye  are  unshaded ;  the  Y-chromosomes  carry  no 
factors  for  eye  color.  (From  Morgan). 


The  Cellular  Basis 


187 


FIG.  62.  RECIPROCAL  OF  CROSS  SHOWN  IN  FIG.  61.  Parents,  white-eyed 
female  and  red-eyed  male ;  F ,  red-eyed  females  and  white-eyed  males 
("criss-cross  inheritance")  ;  F«,  equal  numbers  of  red-eyed  females  and 
males.  The  distribution  of  the  sex-chromosomes  is  shown  as  in  Fig.  61. 
(From  Morgan.) 


i88  Heredity  and  Environment 

linked  with  sex,  such  as  the  color  of  the  eyes  and  of  the  body, 
the  length  of  the  wings,  etc.  A  typical  case  is  shown  in  Figs.  61 
and  62.  The  eye  color  of  this  fly  is  normally  red,  but  mutations 
have  arisen  in  which  the  eye  is  white.  Such  a  mutation  first  ap- 
pears in  males,  though  it  may  later  be  transferred  to  females,  as 
we  shall  see.  If  now  a  white-eyed  male  and  a  red-eyed  female  are 
crossed  all  the  F^s  are  red-eyed,  but  if  these  F/s  are  interbred  all 
the  females  of  F2  have  red  eyes  while  half  of  the  males  have  red 
eyes  and  the  other  half  have  white  eyes  (Fig.  61).  On  the  other 
hand  if  one  of  the  F±  females  of  this  cross  is  bred  with  a  white- 
eyed  male  (Fig.  62,  Fx),  half  of  the  females  of  F2  are  red-eyed 
and  half  are  white-eyed,  and  half  of  the  males  are  red-  eyed  and 
half  are  white-eyed. 

If  now  one  of  these  white-eyed  females  is  bred  with  a  red-eyed 


FIG.  63.  DIAGRAM  OF  INHERITANCE  OF  COLOR  BLINDNESS  THROUGH  THE 
MALE.  A  color  blind  male  (here  black)  transmits  his  defect  to  his  grand- 
sons only.  The  corresponding  distribution  of  the  sex  chromosomes  is 
shown  on  the  right,  the  one  carrying  the  factor  for  color  blindness  being 
black.  Recent  work  shows  that  the  six  chrosomes  of  the  human  male 
are  XY  and  not  XO,  as  shown  in  this  diagram. 


The  Cellular  Basis  189 

male  (Fig.  62,  P)  all  the  females  of  the  Ft  generation  are  red- 
eyed  and  all  the  males  white-eyed  ("criss-cross"  inheritance)  and 
if  these  are  interbred  there  are  produced  in  the  F2  generation 
equal  numbers  of  red-eyed  and  white-eyed  males  and  females 
(Fig.  62). 

The  distribution  of  the  maternal  and  paternal  sex  chromo- 
somes exactly  parallels  this  distribution  of  this  sex-linked  char- 
acter, as  is  shown  in  Figs.  61  and  62,  and  this  proves  that  the 
differential  factors  for  these  characters  are  carried  in  these  sex 
chromosomes. 

By  a  series  of  ingenious  experiments  Foot  and  Strobell  have 
shown  that  the  differential  factors  for  certain  sex-limited  char- 
acters in  insects,  that  is,  characters  which  are  limited  to  one  sex, 
are  not  contained  in  the  "sex  chromosomes,"  and  they  argue  that 
the  differential  factors  for  sex  and  for  sex-linked  characters  can- 
not be  located  in  these  chromosomes.  Their  conclusions  apply 
only  to  sex-limited  and  not  to  sex-linked  characters.  There  is 

Chromosomes 

XX      Parents 

9 

^M 

Gametes 


<Q> 


Gametes 


FIG.  64.  DIAGRAM  OF  INHERITANCE  OF  COLOR  BLINDNESS  THROUGH  THE 
FEMALE.  A  color  blind  female  transmits  her  defects  to  all  her  sons,  to 
half  of  her  granddaughters  and  to  half  of  her  grandsons.  Corresponding 
distribution  of"  sex  chromosomes  on  right.  (After  Morgan.) 


19°  Heredity  and  Environment 

no  doubt  that  the  factors  for  the  determination  of  sex  and  sex- 
linked  characters  are  distributed  in  the  same  way  as  the  "sex 
chromosomes"  are,  and  this  proves  that  these  factors  are  located 
in  the  "sex  chromosomes." 

Haemophilia. — Another  case  of  sex-linked  inheritance  is  found 
in  an  abnormal  condition  in  man  known  as  haemophilia,  which  i? 
characterized  by  a  deficiency  in  the  clotting  power  of  the  blood 
and  consequently  by  excessive  bleeding  after  injury.  "Bleed- 
ers" are  almost  always  males,  though  the  defect  is  always  trans- 
mitted to  a  son  from  his  mother  who  does  not  usually  show  the 
defect  because  it  appears  in  females  only  when  both  parents  were 
affected.  The  manner  of  inheritance  of  this  character  is  exactly 
similar  to  the  inheritance  of  white  eyes  in  Drosophila  and  is  in  all 
probability  due  to  similar  causes. 

Daltonism. — One  of  the  most  striking  cases  of  sex-linked  in- 
heritance is  that  form  of  color  blindness  known  as  Daltonism, 
in  which  the  affected  person  is  unable  to  distinguish  between  red 
and  green.  It  is  known  that  males  are  more  frequently  affected 
than  females,  and  that  color  blindness  is  in  some  way  associated 
with  sex.  It  requires  two  determiners  for  color  blindness,  one 
from  the  father,  the  other  from  the  mother,  to  produce  a  color 
blind  female,  whereas  only  a  single  determiner  is  necessary  to 
produce  a  color  blind  male,  just  as  is  true  of  sex.  The  accom- 
panying diagrams  (Figs.  63,  64)  illustrate  the  method  of  inheri- 
tance of  color  blindness.  J[  represents  the  normal  X-chromo- 
some,  Q  its  absence  (or  rather  the  F-chromosome  since  the  sex 
chromosomes  of  the  human  male  are  XY  and  not  XOQ  and  X 
the  X-chromosome  which  carries  the  factor  for  color  blindness. 

It  will  be  seen  that  a  color  blind  father  and  a  normal  mother 
have  only  normal  children,  but  the  father  transmits  to  his  daugh- 
ters and  not  to  his  sons  the  sex-determiner  which  carries  the 
factor  for  color  blindness.  But  since  color  blindness  does  not 
develop  in  females  unless  it  is  duplex  (i.e.  comes  from  both 
father  and  mother)  whereas  it  develops  in  males  if  it  is  simplex 


The  Cellular  Basis  191 

(i.e.  comes  from  either  parent)  all  the  daughters  of  a  color  blind 
father  and  normal  mother  will  appear  normal  although  carrying 
one  determiner  for  color  blindness,  while  all  the  sons  will  be  nor- 
mal because  they  carry  no  determiner  for  color  blindness.  But 
these  daughters  transmit  to  one-half  of  their  children  the  single 
determiner  for  color  blindness  and  if  any  of  those  receiving  this 
determiner  are  males  they  will  be  color  blind.  Consequently  we 
have  the  curious  phenomenon  of  simplex  color  blindness  appear- 
ing only  in  males  and  being  transmitted  to  them  only  through  ap- 
parently normal  females. 

On  the  other  hand  if  a  female  is  color  blind  she  has  inherited  it 
from  both  father  and  mother,  i.e.,  the  character  in  her  is  duplex, 
and  in  all  of  her  children  by  a  normal  male  the  character  will  be 
simplex;  accordingly  all  of  her  sons  will  be  color  blind  and  all  of 
her  daughters  will  be  normal,  though  carrying  the  simplex  deter- 
miner for  color  blindness. 

Sex-linked  Lethal  Factors. — One  of  the  most  interesting  cases 
of  linkage  is  found  where  early  death  is  linked  with  sex.  In 
Drosophila  a  considerable  number  of  lethal  mutant  factors  have 
been  demonstrated  in  the  X  chromosome  and  all  individuals  in 
which  such  a  factor  is  not  balanced  by  a  normal  allelomorph  die 
early.  All  males  that  receive  such  a  lethal  die,  since  there  is  only 
one  X  chromosome  in  the  male ;  all  homozygous  females  that 
have  the  factor  in  both  X  chromosomes  die,  while  only  those  sur- 
vive that  are  heterozygous  for  this  factor.  Such  a  heterozy- 
gous female  produces  in  equal  numbers  eggs  with  and  without 
the  lethal  factor  and  if  she  is  bred  to  a  normal  male  all  of  the 
daughters  are  viable  though  half  of  them  carry  the  lethal  factor 
in  one  of  the  X  chromosomes,  but  all  of  the  males  that  receive 
the  lethal  factor  are  non-viable  since  the  male  has  only  one  X 
chromosome,  while  all  the  males  that  survive  lack  this  factor 
altogether.  Thus  the  sex  ratio  in  this  case  is  2  females  to  i  male. 
Other  lethal  factors  have  occurred  in  other  chromosomes  of 


/92  Heredity  and  Environment 

Drosophila  but  they  were  first  studied  and  are  most  easily  demon- 
strated in  the  X  chromosome. 

b.  Other  Cases  of  Linkage.  —  In  addition  to  characters  which 
are  sex-linked  other  characters  may  be  bound  together  in  hered- 
ity without  being  linked  with  sex.  Morgan  and  his  associates  have 
found  and  studied  about  four  hundred  mutations  of  Drosophila 
(see  Figs.  101-103),  which  are  inherited  in  four  groups,  all  the 
characters  of  each  group  usually  going  together.  There  have  been 
found  in  the  first  group  140  different  mutant  characters,  in  the 
second  125,  in  the  third  120  and  in  the  fourth  3.  Or  eliminating 
lethal  and  modifying  factors  and  those  of  more  doubtful  location, 
there  remain  188  mutant  genes,  50  of  which  are  in  the  first  group, 
70  in  the  second,  65  in  the  third,  and  3  in  the  fourth.  Correspond- 
ing with  the  number  and  size  of  these  groups  there  are  four  pairs 
of  chromosomes  in  Drosophila,  three  of  which  are  large  and  one  is 
very  small  (Fig.  65).  The  sex  chromosomes  (XX  in  the  female, 
XY  in  the  male)  constitute  one  of  the  large  pairs  and  the  genes  of 
the  characters  which  are  sex-linked  are  located  in  these  chro- 
mosomes ;  the  genes  of  the  second  and  third  groups  of  characters 
are  in  the  other  large  chromosomes,  while  the  fourth  group  of  only 
three  characters  have  their  genes  in  the  very  small  chromosomes 
(Fig.  65).  If  this  interpretation  is  correct,  linkage  is  due  to  the 


FIG.  65.  CHROMOSOMES  (DiPLOio  NUMBER)  OF  Drosophila  melanogaster. 
A.  Female  with  2  X  chromosomes;  B.  Male  with  i  X  and  I  Y;  C.  Ma- 
troclinoii'S  female  (XXY)  resulting  from  non-disjunction  of  the  2  X 
chromosomes  of  the  egg.  (After  Morgan.) 


The  Cellular  Basis  193 

grouping  together  of  certain  genes  in  certain  chromosomes,  there 
are  as  many  groups  of  characters  as  there  are  pairs  of  chro- 
mosomes and  as  long  as  the  chromosomes  preserve  their  identity 
the  linkage  of  genes  in  the  chromosomes  and  of  characters  in  the 
developed  organisms  will  persist. 

c.  "Cross-Overs." — But  linkage  of  inherited  characters  is  not 
quite  so  simple  as  this  statement  would  indicate  for  an  extensive 
study  of  this  phenomenon  in  Drosophila  has  shown  that  while 
characters  are  usually  linked  in  four  constant  groups  this  is  not  al- 
ways true.  For  example  Morgan  has  found  that  when  a  female 
fly  with  white  eyes  and  yellow  wings  is  crossed  to  a  male  with 
red  eyes  and  gray  wings,  the  genes  for  these  characters  being 
linked  together  in  the  X  chromosomes,  all  the  sons  are  yellow 
and  have  white  eyes  while  all  the  daughters  are  gray  and  have  red 
eyes,  gray  wings  and  red  eyes  being  dominant  over  yellow  and 
white;  but  when  these  F^s  are  crossed  about  99  per  cent  of  the 
offspring  show  the  same  linkage  of  the  colors  yellow-white,  gray- 
red,  but  in  i  per  cent  the  linkage  is  yellow-red,  gray-white.  This 
interchange  of  characters  in  the  two  groups,  or  "cross-over"  as 
Morgan  calls  it,  may  be  explained  by  assuming  that  there  has 
been  an  interchange  of  genes  between  the  two  sex  chromosomes 
of  the  female.*  When  the  paired  chromosomes  lie  side  by  side 
in  synapsis  it  is  known  that  they  sometimes  twist  around  each 
other  and  if  in  their  subsequent  separation  each  chromosome 
should  break  at  the  point  where  the  two  cross  and  a  portion  of 
one  chromosome  should  be  joined  to  the  other  one  we  would  have  a 
relatively  simple  explanation  of  the  interchange  or  "cross-over" 
of  genes  and  consequently  of  the  breaking  up  of  the  old  group  of 
characters  and  the  establishment  of  a  new  group  (Fig.  66). 

Similar  interchange  of  characters  takes  place  in  each  of  the 
other  three  groups  of  Drosophila,  and  can  be  explained  in  the 

*  Such  "cross-overs"  occur  only  in  the  female  of  Drosophila,  though  they 
may  occur  in  the  males  of  other  species.  They  occur  when  the  synaptic 
pairs  of  chromosomes  are  long,  slender  threads. 


194. 


Heredity  and  Environment 


same  way.  If  a  pair  of  chromosomes  are  twisted  round  each  other 
at  more  than  one  place  and  are  then  broken  at  these  points  we  get 
double  or  multiple  crossing  over  and  a  corresponding  re-grouping 
of  genes  and  of  characters.  Unless  chromosomes  of  a  pair 
are  very  tightly  twisted  two  cross-overs  will  not  occur  near 
together  and  in  general  the  farther  apart  points  are  in  a  chromo- 
some the  more  likely  is  a  cross-over  to  occur.  If  one  per  cent  of 
"crossing  over"  occurs  the  genes  are  assumed  to  be  one  unit  of 
distance  apart;  if  ten  per  cent,  ten  units,  etc.  On  this  basis  Mor- 
gan and  his  associates  have  constructed  a  "map"  of  each  chromo- 
some of  Drosophila  indicating  the  positions  of  those  genes  which 


d 


FIG.  66.  DIAGRAM  SHOWING  THE  PROBABLE  CHROMOSOMAL  MECHANISM 
BY  WHICH  "CROSSING  OVER"  is  CAUSED.  Pairs  of  chromosomes,  one  from 
the  father,  the  other  from  the  mother,  are  shown  in  synapsis  (a,  b,  c)  and 
in  the  reduction  division  (rf).  Homologous  chromomeres  (or  allelomorphic 
genes)  are  represented  by  the  black  and  white  circles  at  the  same  level. 
In  a  and  b  the  chromosomes  are  shown  "crossing  over";  in  c  they  have 
broken  at  the  point  of  crossing  and  half  of  each  chromosome  is  joined  to 
half  of  the  other  one;  in  d  the  "crossed  over"  chromosomes  are  separating 
in  the  reduction  division  (from  Morgan). 


CHROMOSOME  I 


The  Cellular  Basis 

CHROMOSOME  II     CHROMOSOME  III 


195 


CHROMOSOME  IV 


0.0 

o.& 

1.5 

YELLOW'SCUTE' 
J3ROAD* 
WHITE* 

-2.0, 
0.0 

TELEGRAPH 
STAR* 

0.0 

RDUGHOID*             ! 

5.5 
7.5 

ECHINUS' 
RUBY* 

4.0 
9.0 

EXPANDED 
TRUNCATE* 

10.0 

STAR-IHTENSIFIER 

14.0 

CROSSV'NL'3" 

14.0 

STREAK 

15.0 

SMUDGE 

20.0 

CUT* 

2E.O 

CRKAU-B 

20.0 

DWARFOID 

27.5 

•TAN* 

29.0 

DACH3 

i:§ 

SEPIA* 
HAIRY* 

33.0 
36.0 

VERMILION* 
MINIATURE* 

33.0 

SKI-II* 

32.0 
34.0 

DIVER(2NT 

CREAM-III 

38.5 

DICHAETB* 

44.5 

SARNEi* 

46,5 

BLACK* 

42.0 

45.  C 

SCARLET* 
PtNK* 

m 

FORKED* 
BAR* 

m 

CINNABAR* 
•PURPLE* 

fti 

59.Q 

SPINELESS* 
BITHORAX 

(aASS 

65.0 
70.0 

CLEFT 
BOBBED 

65.6 

70.  Q 
73.5: 

VESTIGIAL* 

LOBE* 
CURVED* 

63.5 
65.5 
67.5 

72,0 

DELTA 
HAIRLESS* 
EBONY* 

WHITE.-OCBLLI* 

85.0 
88.0 

ttINUTE-2 

HUMFX 

8615 

90.0 

«6UGH* 
POINTED  WING 

' 

95.5 

•CLARET* 

98.5 

103.0 
105.0! 

PLEXUS* 

BROWN* 
SPECK* 

101.  a 

MINUTE-23* 

FIG.  67.  MAPS  OF  THE  FOUR  CHROMOSOMES  OF  Drosophila  melanogaster 
showing  positions  of  a  few  of  the  nearly  400  mutant  genes ;  those  that  have 
been  most  accurately  located  are  marked  with  a  star.  (From  information 
furnished  by  Morgan  and  Bridges.)  Figures  of  several  of  these  Droso- 
phila Mutuals  will  be  found  on  pp.  286-288. 


196  Heredity  and  Environment 

have  been  determined  most  accurately  (Fig.  67).  Thus  not  only 
do  they  locate  particular  genes  in  particular  chromosomes  but  they 
are  able  to  locate  the  relative  positions  of  these  genes  in  each 
chromosome.  This  is  in  all  respects  the  most  remarkable  work 
which  has  ever  been  done  in  this  field;  for  the  first  time  it  gives 
us  a  detailed  picture  of  what  Weismann  called  the  "architecture 
of  the  germplasm," — for  the  first  time  it  assigns  to  different 
genes  "a  local  habitation  and  a  name." 

3.  CYTOPLASMIC  INHERITANCE. — The  most  direct  and  the  earli- 
est recognized  correlations  between  the  oosperm  and  the  devel- 
oped animal  are  found  in  the  polarity  and  symmetry  of  the  egg 
cytoplasm  and  of  the  animal  to  which  it  gives  rise. 

(a)  Polarity. — In  all  eggs  there  is  polar  differentiation,  one 
pole,  at  which  the  maturation  divisions  take  place,  being  known 
as  the  animal  pole,  and  the  opposite  one  being  known  as  the 
vegetative  pole.    The  substance  of  the  egg  in  the  vicinity  of  the 
animal  pole  usually  gives  rise  to  the  ectoderm,  or  outer  cell  layer 
of  the  embryo ;  the  portion  of  the  egg  surrounding  the  vegetative 
pole  usually  becomes  the  endoderm  or  inner  cell  layer-    The  axis 
which  connects  these  poles,  the  chief  axis  of  the  egg,  becomes 
the  gastrular  axis  of  the  embryo  and  in  every  great  group  of 
animals  it  bears  a  constant  relationship  to  the  chief  axis  of  the 
adult  animal.    The  polarity  of  the  developed  animal  is  thus  di- 
rectly connected  with  the  polarity  of  the  egg  from  which  it  came 
(Figs.  42,45,  46,47,  68,  69). 

(b)  Symmetry. — In  many  cases  the  symmetry  of  the  developed 
animal  is  foreshadowed  in  the  cytoplasm  of  the  egg.     The  eggs 
of  cephalopods  (Fig.  68)  and  of  insects  (Fig.  69)  are  bilaterally 
symmetrical  while  they  are  still  in  the  ovary ;  in  other  cases,  such 
as  ascidians,  Amphioxus  and  the  frog,  bilateral  symmetry  ap- 
pears immediately  after  fertilization  (Figs.  9,  10,  46,  47),  though 
in  some  of  these  cases  there  is  reason  to  believe  that  the  eggs  are 
bilateral  even  before  fertilization;  in  still  other  cases  bilaterality 
does  not  become  visible  until  later  in  development  and  we  do  not 


The  Cellular  Basis 


197 


now  know  whether  it  is  present  in  earlier  stages  or  not ;  but  wher- 
ever it  can  be  recognized  in  the  earlier  stages  it  is  probable  that  the 
bilateral  symmetry  of  the  egg  becomes  the  bilateral  symmetry  of 
the  developed  animal. 

(c)  Inverse  Symmetry. — In  most  animals  bilateral  symmetry 
is  not  perfect,  certain  organs  being  found  on  one  side  of  the  mid- 
line  and  not  on  the  other,  or  being  larger  or  differently  located  on 


FIG.  68 


FIG.  69 


FIG.  68.  OUTLINES  OF  THE  UNFERTILIZED  EGG  OF  A  SQUID,  Loligo,  show* 
ing  the  polarity  and  symmetry  of  the  egg  with  reference  to  the  axes  of 
the  developed  animal;  d,  dorsal;  v,  ventral;  /,  left;  r,  right;  a,  anterior; 
p,  posterior.  (After  Watase.) 

FIG.  69.  MEDIAN  SECTION  THROUGH  EGG  OF  A  FLY,  Musca,  just  after  fer- 
tilization, showing  the  relations  of  the  polarity  and  symmetry  of  the  egg 
to  the  axes  of  the  developed  animal ;  the  long  axis  of  the  egg  corresponds 
to  the  antero-posterior  axis  of  the  animal ;  d,  dorsal ;  v,  ventral ;  m,  micro- 
pyle  through  which  sperm  enters  the  egg ;  g,  glutinous  cap  over  the  micro- 
pyle ;  r,  polar  bodies ;  p,  egg  and  sperm  nuclei ;  do,  yolk ;  k,  peripheral  layer 
of  protoplasm;  dh,  vitelline  membrane  of  egg;  ch,  chorion.  (After  Kor- 
schelt  and  Heider.) 


198  Heredity  and  Environment 

one  side  as  compared  with  the  other ;  among  all  such  animals  var- 
iations occasionally  occur  which  show  a  complete  reversal  of  these 
asymmetrical  organs,  i.e.,  in  man  the  heart  and  arch  of  the  aorta 
may  occur  on  the  right  side  instead  of  the  left,  the  pyloris  and 
chief  portion  of  the  liver  on  the  left  instead  of  the  right,  etc. 
Among  certain  snails  this  inversion  of  symmetry  may  occur 
regularly  in  certain  species  and  not  in  others,  the  inverse  form 
being  known  as  sinistral  and  the  ordinary  form  as  dextral  (Fig. 
72).  In  these  sinistral  snails,  and  probably  in  all  animals  show- 
ing inverse  symmetry,  the  embryo  is  inversely  symmetrical  and 
every  cleavage  of  the  egg  from  the  first  to  the  last  is  the  inverse 
of  that  which  occurs  in  dextral  snails  (Figs.  70-72).  There  is 
good  reason  to  believe  that  in  such  cases  the  unsegmented  egg  is 
also  inversely  symmetrical  as  compared  with  the  more  usual  type 
(Fig.  70).  In  all  of  these  cases  there  is  a  direct  correspondence 
between  the  polarity  and  symmetry  of  the  oosperm  and  the  polar- 
ity and  symmetry  of  the  developed  animal  (Figs.  68-72). 

(d)  Localization  Pattern. — In  many  animals  the  ectoderm  and 
mesoderm  may  be  traced  back  to  areas  of  peculiar  protoplasm  in 
the  oosperm,  but,  in  addition  to  this,  one  can  recognize  in  the  as- 
cidian  egg  areas  of  peculiar  protoplasm  which  will  give  rise  to 
mesenchyme,  muscles,  nervous  system  and  notochord,  and  these 
substances  are  present  in  the  oosperm  in  the  approximate  posi- 
tions and  proportions  which  they  will  have  in  the  embryo  and 
larva  (Figs.  10,  n,  46-48). 

Indeed  there  are  types  of  localization  of  these  cytoplasmic 
materials  in  the  egg  which  are  characteristic  of  certain  phyla ;  thus 
there  are  the  ctenophore,  the  flat-worm,  the  echinoderm,  the  an- 
nelid-mollusk  and  the  chordate  types  of  cytoplasmic  localization 
(Fig.  73).  The  polarity,  symmetry  and  pattern  of  a  jellyfish, 
starfish,  worm,  mollusk,  insect  or  vertebrate  are  foreshadowed  by 
the  characteristic  polarity,  symmetry  and  pattern  of  the  cyto- 
plasm of  the  egg  either  before  or  immediately  after  fertilization. 
In  all  of  these  phyla,  eggs  may  develop  without  fertilization, 


The  Cellular  Basis  199 

either  by  natural  or  by  artificial  parthenogenesis,  and  in  such  cases 
the  characteristic  polarity,  symmetry  and  pattern  of  the  adult  are 
found  in  the  cytoplasm  of  the  egg  just  as  if  the  latter  had  been 
fertilized.  The  conclusion  seems  to  be  justified  that  these  earliest 
and  most  fundamental  differentiations  which  distinguished  the 
eggs  of  various  phyla  are  not  dependent  upon  the  entering  sperma- 
tozoon. 

Share  of  Egg  and  Sperm  in  Heredity. — All  of  these  corre- 
spondences between  the  polarity,  symmetry  and  pattern  of  the 
egg  and  of  the  developed  animal  are  found  in  the  cytoplasm.  No 
doubt  the  differentiations  of  the  cytoplasm  of  the  egg  as  well  as 
the  peculiar  form  and  structure  of  the  spermatozoon  have  arisen, 
during  the  genesis  of  these  cells,  under  the  influence  of  paternal 
and  maternal  chromosomes  as  well  as  of  the  environment,  just 
as  in  the  differentiation  of  any  tissue  cell;  but  in  the  case  of  the 
spermatozoon  these  cytoplasmic  differentiations  are  lost  when  it 
enters  the  egg,  whereas  those  of  the  egg  persist.  In  short  on- 
togeny begins  in  the  egg  before  fertilization  whereas  the  sperm 
can  influence  ontogeny  only  after,  and  usually  a  considerable 
time  after,  it  enters  the  egg. 

The  fact  remains  that  at  the  time  of  fertilisation  the  poten- 
cies of  the  two  germ  cells  are  not  equal,  the  polarity,  sym- 
metry, type  of  cleavage,  and  the  pattern,  or  relative  positions  and 
proportions  of  future  organs,  being  foreshadowed  in  the  cyto- 
plasm of  the  egg  cell,  while  only  the  differentiations  of  later  de- 
velopment are  influenced  by  the  sperm.  In  short  the  egg  cyto- 
plasm determines  the  early  development  and  the  sperm  and  egg 
nuclei  control  only  later  differentiations. 

We  are  vertebrates  because  our  mothers  were  vertebrates  and 
produced  eggs  of  the  vertebrate  pattern ;  but  the  color  of  our  skin 
and  hair  and  eyes,  our  sex,  stature,  and  mental  peculiarities  were 
determined  by  the  sperm  as  well  as  by  the  egg  from  which  we 
came.  The  chromosomes  of  the  egg  and  sperm  are  the  seat  of 
the  differential  factors  or  determiners  for  Mendelian  characters, 


200 


Heredity  and  Environment 


FIGS.  70,  71,  72.  THE  CAUSE  OF  INVERSE  SYMMETRY  IN  SNAILS.  In  each 
case  the  right-hand  column  represents  dextral  forms,  the  left-hand  column 
sinistral  ones. 


FIG.  70.     NORMAL  AND  INVERSE  SYMMETRY  IN  THE  UNSEGMENTED  EGG 

AND  IN  THE  FlRST  AND  SECOND  CLEAVAGES. 


The  Cellular  Basis 


201 


FIG.  71.  NORMAL  AND  INVERSE  SYMMETRY  OF  THE  30,  4TH,  STH  AND  6TH 
CLEAVAGES.  The  cells  la-id,  2a-2d  and  sa-sd  give  rise  to  all  the  ectoderm ; 
4d  or  M  gives  rise  to  mesoderm ;  A,  B,  C,  D  to  endoderm. 


202 


Heredity  and  Environment 


FIG.  72.  NORMAL  AND  INVERSE  SYMMETRY  IN  LATE  EMBRYOS  AND  ADULT 
STAGES.  In  i,  cross-hatched  area  is  blastopore ;  cells  shaded  by  lines,  meso- 
derm;  other  cells,  endoderm;  the  spiral  twist  of  the  snail  begins  in  oppo- 
site directions  in  the  two  embryos.  In  2,  the  adult  organization  is  shown 
with  all  organs  inversely  symmetrical;  os,  olfactory  organ,  a,  anus;  L, 
lung;  V,  ventricle;  K,  kidney.  In  3,  sinistral  and  dextral  shells  of  adult 
snails  are  shown. 


The  Cellular  Basis  203 

CTENOPHORE  TURBELLARIAN 


eel. 


end. 


EOHINODERM 


end 


ASCIDIAN  I 


end. 


mcs. 


ASCI  WAN  ii 


end. 

FIG.  73.  TYPES  OF  EGG  ORGANIZATION  IN  DIFFERENT  PHYLA;  cross- 
hatched  area,  mesoderm  or  mesenchyme  (mes)  ;  horizontal  lines,  endo- 
derm  (end)  ;  clear  area,  ectoderm  (ect).  In  the  first  four  figures  the 
pattern  of  localization  is  that  which  is  found  at  the  close  of  the  first  cleav- 
age; in  Ascidian  II  the  pattern  is  that  which  is  found  at  the  close  of  the 
second  cleavage;  in  the  annelid  egg  the  localization  of  later  stages  is 
projected  upon  the  egg;  n.p.,  neural  plate;  ch.,  chorda;  e.g.,  cerebral 
ganglion;  v.g.,  ventral  ganglion;  proto.,  prototroch. 


2O4  Heredity  and  Environment 

but  the  general  polarity,  symmetry  and  before  fertilization.  But 
these  egg  characters,  like  any  other  character  of  the  female,  were 
probably  determined  by  chromosomes  derived  from  her  father 
and  mother;  if  so  they  are  Mendelian  characters  inherited  by 
the  mother  through  her  chromosomes  and  carried  over  to  the 
first  filial  generation,  not  as  factors  but  as  developed  characters. 
Such  cases  may  be  called  "maternal  inheritance"  since  the  charac- 
ters come  only  from  the  egg,  or  "preinheritance"  since  these  egg 
characters  are  developed  before  fertilization.  (See  pp.  113,  114.) 
Share  of  Chromosomes  and  Cytoplasm  in  Heredity  and  Devel- 
opment.— It  will  be  observed  that  the  correlation  between  chromo- 
somes and  adult  characters  is  different  in  kind  from  that  between 
the  cytoplasm  of  the  egg  and  adult  characters;  in  the  latter  case 
polarity,  symmetry  and  pattern  of  localization  are  characters  of  the 
same  kind  in  the  egg  and  in  the  adult,  and  the  correspondence  is 
comparatively  close ;  in  the  former  there  is  no  correspondence  in 
kind  between  the  chromosomal  peculiarities  and  the  peculiarities  of 
the  adult.  This  fact  suggests  that  the  chromosomal  organization 
is  more  fundamental  than  that  of  the  cytoplasm;  the  chromo- 
somes contain  the  germplasm,  the  cytoplasm  is  the  somatoplasm ; 
the  chromosomes  are  chiefly  concerned  in  heredity,  the  cytoplasm 
in  development. 

E.    THE  MECHANISM  OF  DEVELOPMENT 

Development  consists  in  the  transformation  of  the  oosperm  into 
the  adult.  What  is  the  mechanism  by  which  this  transformation 
is  effected?  There  is  progressive  differentiation  of  the  germ  into 
the  developed  organism  but  by  what  process  is  this  differentiation 
accomplished  ? 

Many  different  processes  are  concerned  in  embryonic  differ- 
entiation. From  the  standpoint  of  the  cell  the  most  important  of 
these  are  (i)  the  formation  of  different  kinds  of  substances 
in  cells,  (2)  the  localization  and  isolation  of  these  substances, 
(3)  the  transformation  of  these  substances  into  various  struc- 


The  Cellular  Basis  205 

tures  which  are  characteristic  of  the  different  kinds  of  tissue  cells. 
We  shall  here  describe  only  the  first  and  second  of  these  pro- 
cesses which  are  of  more  general  interest  than  the  last. 

i.  The  Formation  of  Different  Substances  in  Cells. — Embry- 
onic differentiation  consists  primarily  in  the  formation  of  dif- 
ferent kinds  of  protoplasm  out  of  the  protoplasm  of  the  germ 
cells.  It  is  plain  that  different  kinds  of  protoplasm  are  present 
in  the  two  germ  cells  before  they  unite  in  fertilization,  but  in  the 
course  of  development  the  number  of  substances  present  and  the 
degree  of  difference  among  them  greatly  increase. 

Actual  observation  shows  that  by  the  interaction  with  one 
another  of  substances  or  parts  originally  present  and  by  their 
reactions  to  external  stimuli  new  substances  and  parts  appear 
which  had  no  previous  existence  just  as  new  substances  result 
from  chemical  reactions.  This  is  "creative  synthesis"  in  general 
science,  epigenesis  in  development.  Differentiations  appear 
chiefly  in  the  cytoplasm  but  only  as  the  result  of  interaction  be- 
tween cytoplasm  and  nucleus.  Similarly,  it  may  be  argued,  smal- 
ler units  of  organization  such  as  chromosomes  or  chromomeres 
do  not  in  themselves  give  rise  to  any  adult  parts,  but  only  as  they 
interact  upon  other  units  are  new  parts  formed. 

In  many  cases  the  first  formation  of  such  new  substances  ap- 
pears in  the  immediate  vicinity  of  the  nucleus  and,  like  assimila- 
tion itself,  this  is  evidently  brought  about  by  the  interaction  of  nu- 
cleus and  cytoplasm.  In  certain  cases  it  can  be  seen  that  the 
achromatin  and  oxychromatin  which  escape  from  the  nucleus 
during  division  take  part  in  the  formation  of  new  substances  in 
the  cell  body,  and  since  the  oxychromatin  is  derived  from  the 
chromosomes  of  the  previous  cell  division,  it  is  probable  that  the 
chromosomes  are  a  factor  in  this  process. 

Weismann  maintained  that  the  chromosomes  and  the  inheri- 
tance units  contained  in  them  undergo  differentiation  by  a  pro- 
cess of  disintegration  and  that  these  disintegrated  units  escape 
into  the  cell  body  and  there  produce  different  kinds  of  cytoplasm 


2o6  Heredity  and  Environment 

in  different  cells.  A  somewhat  similar  view  was  advanced  by 
deVries  in  his  theory  of  "intra-cellular  pangenesis."  However,  as 
we  have  seen  already,  there  is  good  evidence  that  the  chromosomes 
do  not  undergo  progressive  differentiation  in  the  course  of  devel- 
opment; they  always  divide  with  exact  equality,  and  even  in 
highly  differentiated  tissue  cells  their  number  and  form  usually 
remain  as  in  embryonic  cells. 

On  the  other  hand  the  cytoplasm  undergoes  progressive  dif- 
ferentiation, and  when  by  pressure  or  centrifugal  force  it  is 
brought  into  relations  with  other  nuclei  the  differentiations  of 
the  cytoplasm  are  not  altered  thereby,  thus  showing  that  the  dif- 
ferent nuclei  are  essentially  alike  and  that  differentiations  are 
mainly  limited  to  the  cytoplasm.  Thus  the  differentiations  of  cells 
are  not  due  to  the  differentiations  of  their  nuclei,  but  rather  the 
reverse  is  true,  such  differentiations  of  nuclei  as  occur  are  due 
to  differentiations  of  the  cytoplasm  in  which  they  lie.  Never- 
theless differentiations  do  not  take  place  in  the  absence  of  nu- 
clear material,  and  it  seems  probable  that  the  interaction  of  nucleus 
and  cytoplasm  is  necessary  to  the  formation  of  the  new  cytoplas- 
mic  substances  which  appear  in  the  course  of  development. 

2.  Segregation  and  Isolation  of  Different  Substances  m  Cells. 
— But  differentiation  consists  not  only  in  the  formation  of  differ- 
ent kinds  of  substances  in  cells  but  also  in  the  separation  of  these 
substances  from  one  another.  This  separation  is  brought  about 
to  a  great  extent  by  flowing  movements  within  cells  which  are 
associated  especially  with  cell  division. 

In  all  these  processes  of  heredity  and  development  cell  divi- 
sion plays  a  particularly  important  part.  If  cell  divisions  were 
always  exactly  alike  there  could  be  no  initial  difference  between 
the  daughter  cells,  and  unless  acted  upon  by  different  stimuli  all 
cells  would  remain  exactly  alike.  But  there  is  much  evidence 
that  daughter  cells  are  often  unlike  from  the  time  of  their  for- 
mation, and  that  different  stimuli  act  upon  them  to  increase  still 
further  this  initial  difference. 


The  Cellular  Basis  207 

(a)  -Differential  and  Non-differential  Cell  Division — When 
each  half  of  any  dividing  unit  is  like  the  other  half  the  division 
is  non-differential.     So  far  as  we  know  the  divisions  of  all  the 
smallest  elements  of  the  cell  are  of  this  sort;  there  is  no  good 
evidence  that  the  plastosomes,  the  chromomeres,  or  the  chromo- 
somes ever  divide  into  unlike  halves,  though  in  the  maturation 
divisions  the  separation  of  whole  chromosomes  leads  to  the  ap- 
pearance of  a  differential  division  of  the  chromosomes.     But 
while  all  of  the  cell  elements  may  be  supposed  to  grow  and  divide 
into  equivalent  halves  there  may  be  an  unequal  distribution  of 
these  elements  in  cell  division,  so  that  the  two  daughter  cells  are 
unlike.    This  is  what  is  known  as  differential  cell  division  and  it 
plays  a  most  important  part  in  differentiation.    While  the  chro- 
matin  is  equally  distributed  to  the  daughter  cells,  except  in  the 
case  of  the  maturation  divisions,  the  achromatin  and  the  oxy- 
chromatin  of  the  nucleus  are  not  always  distributed  equally  and 
this  is  probably  an  important  factor  in  development.     The  divi- 
sions   of  the  cytoplasm  of  the  egg  are  frequently  differential  and 
such  divisions  are  known  to  play  a  great  part  in  embryonic  differ- 
entiation. 

(b)  Isolation  of  Cytoplasmic  Substances  by  Division  Walls. — 
In  the  differential  divisions  of  the  cytoplasm  unlike  substances 
become  localized  in  certain  parts  of  the  cell  body,  chiefly  by  means 
of  definite  flowing  movements  of  the  cytoplasm,  and  when  cell  di- 
vision occurs  these  substances  become  permanently  separated  by 
partition   walls.     In   this   way   irreversible   differentiations   are 
formed.    If  the  formation  of  partition  walls  is  prevented  the  dif- 
ferent substances  within  the  cell  body  may  freely  commingle,  es- 
pecially during  nuclear  division  when  the  cytoplasmic  movements 
are  especially  active;  in  such  cases  differentiation  may  be  ar- 
rested even  though  nuclear  division  continues.     In  the  develop- 
ing eggs  of  most  animals  partition  walls  between  daughter  cells 
are  necessary  to  prevent  the  commingling  of  different  kinds  of 
substances,  which  are  sorted  by  the  movements  within  the  cell  and 


208  Heredity  and  Environment 

are  isolated  by  the  partition  walls.  In  some  cases,  as  for  example 
in  certain  protozoa,  the  commingling  of  different  kinds  of  pro- 
toplasm within  a  cell  may  be  prevented  by  the  viscosity  of  por- 
tions of  the  protoplasm,  or  by  the  formation  of  intracellular  mem- 
branes, or  by  a  reduction  to  a  minimum  of  the  mitotic  movements 
within  the  cell  by  the  persistence' of  the  nuclear  membrane  dur- 
ing division.  In  general  the  degree  of  differentiation  may  be 
measured  by  the  degree  of  unlikeness  between  different  cells,  and 
by  the  completeness  with  which  the  protoplasm  of  different  cells 
is  kept  from  intermingling. 

(3)  The  Chromosome  Theory  of  Heredity  Applied  to  Embry- 
onic Differentiation. — According  to  the  chromosome  theory  of 
heredity  the  inheritance  factors  are  located  in  the  chromosomes, 
and  the  cytological  evidence  shows  that  chromosomes  always  di- 
vide equally  and  presumably  every  cell  of  an  individual  contains 
the  same  kinds  of  chromosomes  and  the  same  kinds  of  inheritance 
factors.  How  then  is  it  possible  to  explain  embryonic  differen- 
tiation ?  How  can  identical  factors  give  rise  to  different  products 
in  different  cells? 

This  is  evidently  due  to  the  fact  that  while  the  division  of 
chromosomes  is  non-differential,  that  of  the  cell  body  is  often 
differential  and  the  same  'chromosomes  and  genes  acting  upon 
different  kinds  of  cytoplasm  will  produce  different  results.  But 
differential  cell-division  is  the  result  of  definite  movements  in  the 
cytoplasm,  of  definite  orientations  of  spindles  and  cleavage  planes, 
and  ultimately  of  a  definite  polarity  and  symmetry  of  the  cyto- 
plasm. There  is  abundant  evidence  that  these  cytoplasmic  orien- 
tations are  not  the  immediate  results  of  chromosomal  activity  and 
even  if  some  of  them  may  be  the  remote  results  of  such  activity  it 
is  logically  impossible  to  place  all  the  differential  factors  of  de- 
velopment in  non-differentiating  genes. 

On  the  other  hand  if  embryonic  differentiations  are  produced 
by  the  interaction  of  chromatin  and  cytoplasm,  and  if  the  chro- 
matin  does  not  undergo  differentiation,  it  follows  that  some  of  the 


The  Cellular  Basis  209 

differential  factors  of  heredity  and  development  must  be  located 
in  the  cytoplasm.  Such  factors  would  probably  not  be  genes  and 
would  not  be  transmitted  in  Mendelian  fashion, -but  they  would 
need  to  be  present  in  the  cytoplasm  from  the  very  beginnings  of 
ontogeny.  They  need  not  be  numerous — in  fact  they  are  prob- 
ably few  in  number — but  they  are  absolutely  indispensable  to 
development.  If  a  few  orientating  differentiations  such  as 
polarity  and  symmetry  are  present  in  the  cytoplasm  at  the  be- 
ginning of  ontogeny  all  other  differentiations  of  development  can 
be  explained  as  due  to  the  interaction  of  non-differentiating 
genes  on  different  parts  of  this  cytoplasm,  but  there  is  no  mecharr- 
ism  by  which  embryonic  differentiations  could  come  from  the 
action  of  non-differentiating  genes  on  a  homogeneous  cytoplasm. 
The  genes  or  Mendelian  factors  are  undoubtedly  located  in  the 
chromosomes  and  they  are  sometimes  regarded  as  the  only  dif- 
ferential factors  of  development,  but  if  this  w"ere  true  these  genes 
would  of  necessity  have  to  undergo  differential  division  and  dis- 
tribution to  the  cleavage  cells;  since  this  is  not  true  it  must  be 
that  some  of  the  differential  factors  of  development  lie  outside 
of  the  nucleus  and  if  they  are  inherited,  as  most  of  these  early 
orientations  are,  they  must  lie  in  the  cytoplasm. 

SUMMARY 

All  the  phenomena  of  life,  including  heredity  and  development, 
are  cellular  phenomena  in  that  they  include  only  the  activities  of 
cells  or  of  cell  aggregates.  The  cell  is  the  ultimate  independent 
unit  of  organic  structure  and  function.  In  sexual  reproduction 
the  only  living  bond  between  one  generation  and  the  next  is  found 
in  the  sex  cells  and  all  inheritance  must  take  place  through  these 
cells.  Inherited  traits  are  not  transmitted  from  parents  to  off- 
spring but  the  germinal  factors  or  causes  are  transmitted,  and  under 
proper  conditions  of  environment  these  give  rise  to  developed  char- 
acters. Every  oosperm  as  well  as  every  developed  organism  differs 
more  or  less  from  every  other  one  and  this  remarkable  condition  is 


2io  Heredity  and  Environment 

brought  about  by  extremely  numerous  permutations  in  the  distri- 
bution of  the  chromosomes  of  the  sex  cells  in  maturation  and  ferti- 
lization. Sex  is  an  inherited  character  dependent,  primarily,  upon 
an  alternative  distribution  of  certain  chromosomes  to  the  germ  cells. 
There  is  much  evidence  that  the  factors  for  all  sorts  of  Mendelian 
characters  are  associated  with  the  chromosomes.  The  differen- 
tiation of  the  oosperm  into  the  developed  organism  is  accom- 
plished in  part  by  the  interaction  of  chromosomes  and  cytoplasm 
which  leads  to  the  formation  of  new  materials,  and  in  part  by  the 
segregation  and  localization  of  these  materials  in  definite  cells. 

Germ  cells  and  probably  all  other  kinds  of  cells  are  almost 
incredibly  complex.  We  know  that  former  students  of  the  cell 
greatly  underestimated  this  complexity  and  there  is  no  reason  to 
suppose  that  we  have  fully  comprehended  it.  What  Darwin  said 
of  the  entire  organism  may  now  be  said  of  every  cell :  It  "is  a  mi- 
crocosm— a  little  universe,  formed  of  a  host  of  self -propagating 
organisms,  inconceivably  minute  and  numerous  as  the  stars  in 
heaven." 


CHAPTER  IV 
INFLUENCE  OF  ENVIRONMENT 


CHAPTER  IV 


INFLUENCE  OF  ENVIRONMENT 

The  development  of  an  individual  or  the  evolution  of  a  race 
is  dependent  upon  the  interaction  of  two  sets  of  factors  or  causes, 
the  intrinsic  and  the  extrinsic.  The  former  are  represented  by 
the  organization  of  the  germinal  protoplasm,  the  latter  by  all 
other  conditions ;  the  former  are  known  as  heredity  or  constitu- 
tion, the  latter  as  environment  or  education;  or  in  the  words  of 
Galton,  these  two  sets  of  factors  may  be  called  "nature"  and  "nur- 
ture." The  great  problem  of  development  is  the  unraveling  of 
these  two  factors,  the  assignment  of  its  true  value  to  each,  and 
the  ultimate  control  of  development  so  far  as  this  may  be  possible 
through  the  knowledge  thus  gained. 

A.    RELATIVE  IMPORTANCE  OF  HEREDITY 
AND  ENVIRONMENT 

The  distinction  between  these  two  factors  of  development  is 
generally  recognized  and  the  question  of  the  relative  importance 
of  the  two  has  been  discussed  for  ages.  Which  is  the  more  im- 
portant, constitution  or  environment?  What  characteristics  are 
due  to  nature  and  what  to  nurture?  To  what  extent  is  man  the 
creature  of  heredity,  to  what  extent  the  product  of  education? 
The  old  question  "Which  of  you  by  taking  thought  can  add  one 
cubit  to  his  stature,"  is  a  vital  question  to-day.  To  what  extent 
may  nature  be  modified  by  nurture  ?  To  what  extent  may  educa- 
tion make  up  for  deficiencies  of  birth? 

i.  Former  Emphasis  on  Environment. — Formerly  very  great 

213 


214  Heredity  and  Environment 

emphasis  was  placed  upon  influences  of  environment  in  phylogeny 
and  ontogeny.  From  the  earliest  times  it  has  been  believed  that 
species  might  be  transmuted  by  environmental  changes  and  that 
even  life  itself  might  arise  from  lifeless  matter  through  the  in- 
fluence of  favorable  extrinsic  conditions.  If  environment  could 
exert  so  great  an  influence  on  the  origin  of  species  or  even  of 
life  itself  much  more  could  it  affect  the  process  of  development  of 
the  individual.  It  is  still  popularly  supposed  that  complexion  is 
dependent  upon  the  intensity  of  light,  and  stature  upon  the  quan- 
tity and  quality  <of  food,  that  sex  is  determined  by  food  or  tem- 
perature, mentality  by  education,  and  that  in  general  individual 
peculiarities  are  due  to  environmental  differences. 

Many  philosophers  of  the  seventeenth  and  eighteenth  centur- 
ies taught  that  man  was  the  product  of  environment  and  educa- 
tion and  that  all  men  were  born  equal  and  later  became  unequal 
through  unequal  opportunities.  Decartes  begins  his  famous  "Dis- 
course on  Method"  with  these  words: 

"Good  sense  is,  of  all  things  among  men,  the  most  equally  dis- 
tributed .  .  .  The  diversity  of  our  opinions  does  not  arise  from 
some  being  endowed  with  a  larger  share  of  Reason  than  others, 
but  solely  from  this,  that  we  conduct  our  thoughts  along  different 
ways,  and  do  not  fix  our  attention  on  the  same  objects." 

Similar  views  were  expressed  by  Rousseau  and  Diderot,  and 
especially  by  John  Locke  and  Adam  Smith.  The  Declaration 
of  Independence  merely  reflected  the  spirit  of  the  age  in  which 
it  was  written  when  it  held  this  truth  to  be  self  evident,  "that 
all  men  are  created  equal."  The  equality  of  man  has  always 
been  one  of  the  foundation  stones  of  democracy.  Upon  this 
belief  in  the  natural  equality  of  all  men  were  founded  systems 
of  theology,  education  and  government  which  hold  the  field 
to  this  day.  Upon  the  belief  that  men  are  made  by  their  en- 
vironment and  training  rather  than  by  heredity  are  founded  most 
of  our  social  institutions  with  their  commands  and  prohibitions, 
their  rewards  and  punishments,  their  charities  and  corrections, 
their  care  for  the  education  and  environment  of  the  individual  and 


Influence  of  Environment  215 

their  disregard  of  the  inheritance  of  the  race.  To  a  large  extent 
civilization  itself  means  good  environmental  conditions,  and  the 
advance  of  civilization  means  improvement  of  environment. 

2.  Present  Emphasis  on  Heredity. — On  the  other  hand  modern 
studies  in  genetics,  are  emphasizing  the  immense,  the  overwhelm- 
ing importance  of  heredity,  in  both  phylogeny  and  ontogeny.    No 
one  now  takes  seriously  the  assertion  that  life  can  be  experi- 
mentally produced  at  the  present  time  from  non-living  matter.    It 
is  evident  that  the  artificial  production  of  life  is  a  much  more 
difficult  problem  than  was  once  supposed,  and  it  may  be  an  in- 
soluble problem.   The  first  flush  of  enthusiasm  over  experimental 
methods  in  biology  led  to  the  expectation  that  we  would  soon  be 
making  species  by  the  process  of  experimental  evolution,  but  the 
results  of  two  or  three  decades  of  such  experimental  work  have 
been  somewhat  disappointing.     Inherited  variations  do  appear, 
incipient  species  arise,  but  there  is  very  little  evidence  to  show 
that  they  appear  in  response  to  environmental  changes  and  at 
present  we  have  no  means  of  controlling  such  variations.    Belief 
in  the  omnipotence  of  environment  for  the  evolution  of  species 
has  steadily  waned  in  recent  years,  while  a  belief  in  the  intrinsic 
causes  of  evolution,   such  as  the  mutation  theory  and  ortho- 
genesis, has  increased. 

In  ontogeny  also  the  environmental  or  extrinsic  factors  of  de- 
velopment have  been  relegated  to  a  subordinate  place,  while  the 
intrinsic  or  hereditary  factors  appear  more  important  than  ever. 
The  old  view  that  men  are  chiefly  the  product  of  environment  and 
training  is  completely  reversed  by  recent  studies  of  heredity.  The 
modifications  which  may  be  produced  by  environment  and  educa- 
tion are  small  and  temporary  as  compared  with  those  which  are 
determined  by  heredity. 

3.  Both  Indispensable. — These  conclusions  are,  in  the  main, 
well  founded.     The  evidence  of  the  tremendous  importance  of 
heredity  is  so  complete  that  we  may  rest  assured  that  thinking 
men  will  never  again  return  to  the  position  which  prevailed  until 


216  Heredity  and  Environment 

a  few  years  ago  regarding  the  all-importance  of  environment. 
And  yet  there  is  danger  of  going  too  far  in  the  opposite  direction. 
Neither  environment  nor  heredity  is  all-important,  but  both  are 
necessary  to  development.  The  germ  cells  with  all  their  inherent 
possibilities  would  forever  remain  germ  cells  were  it  not  for 
environmental  stimuli.  The  realization  of  germinal  possibilities 
is  dependent  upon  the  responses  of  the  germ  to  environmental 
stimuli,  and  although  heredity  is  a  relatively  constant  factor  while 
environment  is  a  more  variable  one,  nevertheless  the  two  are  in- 
dispensable to  development.  Only  by  experiment  can  the  relative 
importance  of  heredity  and  environment  in  development  be  de- 
termined. Extensive  experiments  have  been  made  within  recent 
years  on  developing. animals  and  plants  in  order  to  discover  the 
factors  involved  in  development,  and  the  modifications  which  may 
thus  be  produced  are  very  striking. 

B.  EXPERIMENTAL  MODIFICATIONS  OF  DEVELOPMENT 

The  study  of  development  under  experimental  conditions  has 
given  rise  to  a  new  branch  of  biology,  viz.,  experimental  embry- 
ology or  the  physiology  of  development.  By  changes  in  environ- 
mental conditions  notable  modifications  may  be  produced  in  adult 
organisms,  but  these  modifications  are  much  greater  when  the 
changed  environment  acts  on  the  organism  during  the  period  of  its 
development. 

I.    DEVELOPMENTAL  STIMULI 

It  is  by  no  means  easy  to  define  such  general  terms  as  "environ- 
ment," "stimulus,"  and  "response."  In  its  common  use  "environ- 
ment" means  all  that  lies  outside  the  individual,  if  it  is  defined 
from  the  standpoint  of  the  entire  organism.  But  from  the  stand- 
point of  an  organ  or  cell  it  is  the  surrounding  organs,  cells  or 
fluids  of  the  body;  the  latter  may  be  defined  as  "internal  envi- 
ronment." If  developmental  stimuli  arise  outside  the  organism 


Influence  of  Environment  217 

they  are  plainly  extrinsic  or  environmental,  but  if  they  arise 
within  the  organism  they  are  said  to  be  intrinsic  though  they 
may  be  due  to  changes  in  the  "internal  environment." 

Stimuli  are  chiefly  energy  changes  of  a  physical  or  chemical 
nature.  A  stimulus  to  which  an  adult  organism  responds  by 
movements  or  other  activities  may  call  forth  or  inhibit  develop- 
mental responses  when  applied  to  germ  cells  or  embryos. 

These  developmental  stimuli  may  be  classed  as : 

1.  Physical  stimuli  including  the  following,  (a)  mechanical, 
(b)  thermal,  (c)  electrical,  (d)  radiant,  (e)  light,  (f)  density 
of  medium,  (g)  gravity  and  centrifugal  force,  etc. 

2.  Chemical  stimuli  include  the  action  of  (a)  substances  found 
in  normal  development,  such  as  oxygen,  carbonic  acid,  water, 
food,  secretions  of  ductless  glands,  etc.  and  (b)  substances  not 
found  in  normal  development,  such  as  various  salts,  acids,  alkalis, 
alcohol,  ether,  tobacco,  etc. 

3.  General  vs.  Specific  Stimuli. — In  general  the  action  of  these 
stimuli  during  development  does  not  call  forth  a  perfectly  specific 
and  definite  response  of  the  organism;  various  stimuli  may  pro- 
duce the  same  result.     Thus  artificial  parthenogenesis  has  been 
produced  by  almost  every  stimulus  named,  and  weakened  or  re- 
tarded development  is  produced  by  many  different  stimuli. 

By  the  elimination  of  certain  of  these  stimuli  which  are  normally 
present  or  by  introducing  stimuli  which  are  not  usually  present 
very  important  and  even  profound  changes  in  development  may 
be  produced.  In  this  way  animals  have  been  formed  which  are 
turned  inside  out,  or  side  for  side,  or  in  which  heads  or  nervous 
systems  or  muscles  or  backbones  are  lacking,  or  in  which  the 
various  organs  are  not  found  in  normal  positions.  In  this  way 
dwarfs  and  giants  and  one-eyed  monsters  as  well  as  all  sorts 
of  double  and  partial  embryos  have  been  formed.  In  general 
monstrous  and  defective  forms  of  development  are  due  to  altera- 
tions of  the  normal  environment  rather  than  to  defective  heredity. 


218  Heredity  and  Environment 

II.     DEVELOPMENTAL  RESPONSES 

The  character  of  developmental  responses  to  stimuli  depends 
primarily  upon  (a)  the  nature  of  the  organism  and  (b)  the  stage 
of  development  at  which  the  stimulus  acts.  Modifications  are 
more  easily  produced  and  are  more  profound  during  cell  division 
than  during  intervening  periods  and  at  early  stages  of  develop- 
ment than  at  later  ones ;  indeed  conditions  which  have  no  serious 
effect  on  an  adult  organism  may  greatly  modify  the  development 
of  an  embryo  or  germ  cell. 

i.  Modifications  of  Germ  Cells  before  Fertilization. — It  has 
been  found  by  many  investigators  that  development  may  be  pro- 
foundly changed  by  influences  acting  upon  the  germ  cells  before 
fertilization.  In  general  environmental  changes  acting  during  the 
growth  of  eggs  or  spermatozoa  and  especially  during  their  matura- 
tion may  produce  marked  changes  in  development  though  rarely 
if  ever  in  heredity.  Tower  maintained  that  unusual  conditions 
of  temperature  and  humidity  during  the  later  stages  of  oogenesis 
and  spermatogenesis  may  lead  to  the  production  of  new  races  in 
the  case  of  the  potato  beetle  (Fig.  100)  and  MacDougal's  experi- 
ments on  plants  led  him  to  the  conclusion  that  chemical  substances 
may  influence  the  ovules  so  as  to  change  the  hereditary  character 
of  the  plant.  Other  workers  have  failed  to  confirm  these  results 
and  it  is  doubtful  whether  these  changes  in  hereditary  constitu- 
tion were  caused  by  the  changed  environment.  Bardeen  and  the 
Hertwigs  have  shown  that  great  monstrosities  may  be  produced 
if  X-rays,  radium  or  various  chemical  substances  are  allowed  to 
act  on  spermatozoa  before  fertilization,  but  there  is  no  evidence 
that  these  changes  are  inherited. 

Effects  of  Alcohol. — Stockard  subjected  adult  male  and  fe- 
male guinea-pigs  to  the  fumes  of  alcohol  for  some  time  before 
breeding  them  and  then  studied  the  effects  of  this  drug  on  their 
offspring.  He  finds  that  the  influence  of  alcohol  on  the  sperma- 
tozoa is  as  deleterious  as  when  acting  on  the  ova  and  that  it  pro- 
duces sterility,  or  greatly  reduced  fertility,  a  great  excess  of  still- 


Influence  of  Environment  219 

births,  and  weak  and  sickly  offspring  (Fig.  74).  He  and  Papani- 
colaou  have  studied  the  offspring  of  alcoholized  parents  to  the 
fourth  filial  generation  and  while  the  deleterious  effects  ultimately 
disappear  they  attribute  this  to  the  elimination  of  those  germ 
cells,  embryos  and  developed  individuals  that  were  most  injured 
and  to  the  introduction  of  normal  germplasm  by  crossing  with  un- 
treated animals ;  consequently  the  final  survivors  may  be  stronger 
and  more  vigorous  than  the  controls  in  which  the  weak  are  pre- 
served along  with  the  strong. 

Pearl  found  that  the  offspring  of  alcoholized  chickens  were  on 
the  whole  stronger  than  those  from  normal  animals  and  he 
attributes  this  to  the  elimination  of  weaker  germ  cells  and  em- 
bryos, so  that  only  the  most  sturdy  survive.  The  work  of  both 
Stockard  and  Pearl  leaves  no  grounds  for  doubting  that  alcohol 
kills  or  injures  many  germ  cells  and  Stockard  has  demonstrated 
that  such  injured  cells  may  give  rise  to  defective  individuals  and 
that  this  injury  may  persist  through  two  or  three  generations. 
The  facts  that  spermatozoa  are  affected  even  more  than  ova  and 
that  the  injury  persists  to  the  third  filial  generation  show  that  the 
chromatin  of  these  cells  is  injured.  Undoubtedly  chromatin  as 
well  as  cytoplasm  may  be  injured  by  various  unfavorable  condi- 
tions, and  if  the  injury  is  not  too  great  it  may  persist  through 
several  generations  and  may  cause  defective  development ;  but  this 
is  probably  a  different  thing  from  the  "inheritance  of  an  acquired 
character" ;  its  effects  are  seen  not  in  particular  characters  but  in 
a  general  weakening  of  development;  not  in  imitative  changes  in 
genes,  but  in  their  temporary  injury. 

In  venturing  to  apply  Stockard's  discoveries  to  human  beings 
it  should  not  be  forgotten  that  his  guinea  pigs  were  alcoholized 
to  a  degree  far  greater  than  ever  occurs  in  man.  Some  of  them 
that  were  five  years  old  had  been  kept  intoxicated  for  more  than 
four  years.  It  is  probable  that  the  use  of  alcoholic  beverages  never 
produces  such  serious  effects  on  germ  cells  as  in  the  case  of  these 
guinea  pigs  which  were  compelled  to  inhale  the  fumes  of  strong 


220 


Heredity  and  Environment 


alcohol  throughout  the  greater  part  of  life.  Elderton  and  Pear- 
son made  a  mathematical  study  of  children,  approximately  nine 
years  old,  from  temperate  and  from  intemperate  parents  and  they 
concluded  that  parental  alcoholism  had  practically  no  effect  upon 
them.  However,  the  more  serious  the  injury  to  germ  cells  the 
sooner  they  die  and  it  may  be  that  children  that  survive  for 
nine  years  come  from  germ  cells  that  were  least  injured,  while 
those  that  were  more  seriously  injured  produce  individuals  that 
died  before  or  shortly  after  birth. 


FIG.  74.  DWARFED  GUINEA-PIGS  ON  THE  LEFT  AND  NORMAL  ONES  ON  THE 
RIGHT.  All  are  of  approximately  the  same  age  though  the  normal  ones 
are  nearly  twice  the  weight  of  the  dwarfs.  The  normals  came  from  nor- 
mal parents,  the  dwarfs  from  a  normal  mother  and  an  alcoholic  father; 
the  dwarfing  has  therefore  been  produced  by  the  influence  of  alcohol  on 
the  spermatozoa.  (From  Stockard.) 


Influence  of  Environment  221 

Hoppe  believes  that  a  single  drunken  debauch  may  so  injure 
the  germ  cells  of  man  as  to  produce  abnormal  and  defective  off- 
spring, though  this  is  by  no  means  proved;  while  Hertwig  con- 
cludes that  the  great  prevalence  of  the  drug  habit  may  seriously 
affect  the  germ  cells  and  their  subsequent  development.  Forel 
has  for  many  years  maintained  that  one  of  the  most  serious  causes 
of  human  malformations  and  degenerations  is  to  be  found  in  the 
effect  of  alcohol  on  the  germ  cells,  especially  at  the  time  of  con- 
ception. 

2.  Modifications  During  Fertilization  Stages. — Environmental 
changes  acting  during  fertilization  may  cause  more  than  one  sper- 
matozoon to  enter  the  egg  or  may  injure  the  egg  or  sperm;  in 
either  case  the  resulting  development  is  abnormal.    Where  two  or 
more  spermatozoa  enter  the  egg  the  nuclear  divisions  are  usually 
abnormal,  as  Boveri  has  shown  in  the  case  of  the  sea  urchin;  the 
distribution  of  chromosomes  to  different  cleavage  cells  is  unequal 
and  such  cells  do  not  undergo  typical  development,  while  the 
embryo  or  larva  produced  is  not  capable  of  continued  life.     In 
cases  where  an  egg  is  fertilized  by  a  spermatozoon  belonging  to 
a  different  phylum  or  class    (heterogeneous   fertilization)    the 
foreign  sperm,  after  stimulating  the  egg  to  begin  development, 
may  itself  die  or  remain  inactive,  in  which  case  the  hereditary 
traits  which  develop  are  those  of  the  mother  only.    In  many  ani- 
mals unfertilized  eggs  may  be  stimulated  to  begin  development  by 
a  great  variety  ,of  changes  in  the  medium,  all  such  cases  being 
known  as  ."artificial  parthenogenesis." 

3.  Modifications  of  Development  after  Fertilisation. — Envi- 
ronmental changes,  acting  upon  the  oosperm  after  fertilization,  or 
upon  the  embryo,  may  produce  an  almost  infinite  variety  of  ab- 
normal types  of  development,  but  so  far  as  known  none  of  these 
modifications  becomes  hereditary.    It  seems  probable  that  changes 
in  hereditary  constitution „ take  place  in  the  main  before  fertiliza- 
tion and  especially  during  the  maturation  divisions  and  all  changes 
that  affect  germ  cells  must  of  course  occur  in  the  "germ  track." 


222 


Heredity  and  Environment 


Isolation  of  Cleavage  Cells. — If  the  cleavage  cells  are  separated 
from  one  another  in  the  2-cell  or  4-cell  stage  each  of  them  may 
give  rise  to  an  entire  animal  (Fig.  75)  ;  in  this  way  two  com- 
plete animals  may  be  derived  from  a  single  egg  of  a  star-fish  or 
sea-urchin,  of  an  amphioxus,  and  of  several  other  animal  types. 
If  the  frog's  egg  is  turned  upside  down  in  the  2-cell  stage,  double- 
headed  or  double-bodied  embryos  may  result  (Fig.  76).  In  such 
cases  each  cleavage  cell  is  said  to  be  totipotent,  that  is,  it  is  ca- 
pable of  giving  rise  to  an  entire  animal. 


FIG.  75.  DWARF  AND  DOUBLE  EMBRYOS  OF  Amphioxus.  A,  isolated  blas- 
tomere  of  the  2-cell  stage  segmenting  like  an  entire  egg.  B,  twin  gastrulae 
from  a  single  egg.  C,  double  cleavage  resulting  from  partial  separation  of 
the  first  two  cleavage  cells.  D,  E,  F,  double  gastrulae  arising  from  such 
forms  as  C.  (From  Wilson.) 


Influence  of  Environment 


223 


On  the  other  hand  in  certain  animal  phyla  such  as  the  cteno- 
phores,  mollusks,  annelids  and  ascidians  isolated  cleavage  cells 
give  rise  only  to  part  of  an  animal ;  in  this  way  one  may  get  a 
right  or  left  half  of  an  animal  (Fig.  77)  from  right  or  left  cleav- 
age cells;  an  anterior  half  (Fig.  78),  or  a  posterior  half  (Fig. 
79)  from  anterior  or  posterior  cleavage  cells; 'or  any  one  of  the 
cells  of  the  4-cell  stage  may  produce  the  corresponding  quarter 
of  an  entire  animal.  Such  cases  are  known  as  "mosaic  devel- 
opment." 

There  has  been  much  discussion  among  biologists  as  to  the 


FIG.  76.  DOUBLE  EMBRYOS  OF  FROG  DEVELOPED  FROM  EGGS  INVERTED  WHEN 
IN  THE  2-CELL  STAGE.  A,  twins  with  heads  turned  in  opposite  directions. 
B,  twins  united  back  to  back.  C,  twins  united  by  their  ventral  sides. 
D,  double  headed  tadpole.  (From  Wilson  after  O.  Schultze.) 


224 


Heredity  and  Environment 


FIG.  77.  HALF  AND  THREE-QUARTER  EMBRYOS  OF  Styela.  np,  nerve  plate ; 
nt,  nerve  tube;  E,  eye;  mch,  mesenchyme;  ms,  muscle;  ch,  notochord.  A, 
right  half-blastula  which  developed  after  the  left  half  of  the  egg,  A3Ba,  had 
been  killed.  B,  left  half  larva  from  the  two  left  cells  of  the  4-cell  stage,  the 
right  cells,  AZB,,  having  been  killed.  The  muscle  cells  (stippled)  occur 
only  on  one  side  of  the  notochord.  D,  three-quarter  larva,  the  left  an- 
terior cells  having  been  killed.  E,  F,  three-quarter  larvae,  the  right  pos- 
terior cell  B,  having  been  killed. 


Influence  of  Environment 


225 


E 


FIG.  78.  ANTERIOR  HALF-EMBRYOS  OF  Styela,  the  posterior  cells  having 
been  killed  in  the  4-cell  stage.  Neural  plate,  eye-spots  and  chorda  cells 
are  present  but  no  muscle  cells  or  tail. 


226 


Heredity  and  Environment 


FIG.  79.  POSTERIOR  HALF-EMBRYOS  OF  Styela,  the  anterior  cells  having 
been  killed  in  the  4-cell  stage.  Muscle  cells  and  intestinal  cells  are  present 
but  no  portion  of  neural  plate  or  chorda. 

meaning  of  these  results.  On  the  one  hand  it  has  been  said  that 
the  totipotence  of  any  one  of  the  first  four  cleavage  cells  proves 
that  all  of  these  cells  are  alike  and  that  they  have  not  yet  begun 


Influence  of  Environment  227 

to  differentiate.  On  the  other  hand  it  is  said  that  a  part  of  an 
egg  may  give  rise  to  a  whole  animal  for  the  same  reason  that 
parts  of  certain  adult  animals  may  do  the  same  thing,  viz.,  because 
they  have  the  power  of  regeneration.  However  there  are  many 
animals  which  are  incapable  of  regenerating  lost  parts  of  their 
bodies,  and  similarly  there  are  cases  in  which  part  of  an  egg 
cannot  give  rise  to  a  whole  animal.  The  evidence  available  at 
present  favors  the  view  that  in  cases  where  one  of  the  cleavage 
cells  is  capable  of  giving  rise  to  a  whole  animal  there  is  a  greater 
capacity  of  regeneration  or  regulation,  and  possibly  also  a  lower 
degree  of  initial  differentiation,  than  in  those  cases  in  which  part 
of  an  egg  is  capable  of  producing  only  part  of  an  animal. 

Effects  of  Centrifugal  Force. — If  the  fertilized  egg  is  whirled 
rapidly  on  a  centrifugal  machine  it  may  be  subjected  to  a  pressure 
several  thousand  times  that  of  gravity.  Under  such  conditions 
the  heavier  particles  are  thrown  to  one  side  of  the  egg  and  the 
entire  substance  of  the  egg  becomes  stratified  into  layers  or  zones. 
In  the  ascidian  egg,  where  the  different  kinds  of  protoplasm  give 
rise  to  different  tissues  and  organs,  this  rearrangement  of  the  egg 
substances  may  lead  to  a  marked  dislocation  of  organs;  the 
animal  may  be  turned  inside  out,  having  the  endoderm  on  the 
outside  and  its  ectoderm  or  skin  on  the  inside,  etc.  (Fig.  80). 
On  the  other  hand  in  some  mollusks  and  echinoderms  the  devel- 
opment of  centrifuged  eggs  is  practically  normal.  In  the  for- 
mer case  the  formative  substances  were  dislocated ;  in  the  latter 
they  were  probably  not. 

Double  Monsters  and  Identical  Twins. — If  the  cleavage  cells 
are  only  partially  separated  they  may  produce  animals  which 
are  partially  separated,  such  as  Siamese  twins,  two-headed  forms, 
etc.  (Figs.  75,  76).  Or  these  double  monsters  may  be  produced 
by  division  or  budding  of  the  embryo  at  a  later  stage  of  develop- 
ment. In  the  human  species,  no  less  than  in  other  animals,  all 
sorts  of  double  monsters  may  be  formed  in  this  way  by  the  par- 
tial division  of  a  single  egg  or  embryo  (Fig.  81).  If  the  division 


228 


Heredity  and  Environment 


eel 


FIG.  80.  Two  LARVAE  OF  Styela  which  were  centrifuged  in  the  4-cell 
stage  thereby  changing  the  position  of  various  organ-forming  substances. 
Nervous  system  (ns),  eyes  (E),  notochord  (ch)  and  muscles  (ms)  have 
been  displaced,  and  the  larva  has  been  turned  inside  out,  the  endoderm 
(end)  being  outside  and  the  ectoderm  (ect)  inside. 

is  slight  the  developed  individual  may  show  only  the  beginnings 
of  a  division  into  two,  as  in  two-headed  forms ;  if  the  division  of 
the  egg  or  embryo  is  complete  two  separate  and  perfect  individ- 
uals may  be  formed  from  an  originally  single  oosperm.  When  two 
individuals  are  formed  from  a  single  egg  they  have  exactly  the 
same  heredity  and  accordingly  they  are  always  of  the  same  sex 
and  are  so  similar  in  appearance  that  they  are  known  as  "identical" 
or  "duplicate"  twins  (Fig.  81,  right  end).  On  the  other  hand 
twins  which  develop  from  different  eggs  do  not  have  the  same 
heredity  and  may  differ  in  sex  as  well  as  in  other  features ;  they 
are  known  as  "fraternal"  twins. 

Other  Monstrous  Forms. — If  the  temperature  or  density  of  the 
surrounding  medium  is  altered  during  the  gastrula  stages  the 
endoderm  may  be  caused  to  turn  out  instead  of  in  (exogastrula), 
thus  producing  an  animal  which  is  turned  inside  out  (Fig.  82). 
In  other  cases  (vertebrates)  the  gastrula  mouth  may  fail  to  close, 
thus  producing  animals  in  which  the  spinal  cord  and  vertebral 
column  are  split  in  two  (spina  bifida)  ;  or  the  brain  may  be  forced 
outside  of  the  head  or  may  be  lacking  altogether  (anencephaly). 


Influence  of  Environment 


229 


0.  H» 

FIG.  81.  DIAGRAM  SHOWING  THE  DIFFERENT  TYPES  OF  UNION  OF  DOUBLE 
HUMAN  MONSTERS,  each  being  produced  by  a  partial  division  of  a  single 
egg  or  embryo.  If  the  division  is  a  complete  one,  duplicate  twins  are 
formed,  as  shown  by  the  figures  at  the  right  end  of  each  line.  (From 
Wilder.) 

In  some  cases  eyes  are  wholly  lacking,  in  others  the  two  eyes  fuse 
together  into  a  single  one  as  in  the  fabled  Cyclops  (Fig.  83). 
Practically  all  such  cases  of  monstrous .  development  are  due  to 


End 


FIG.  82.  EXOGASTRULA  OF  Crepidula.  The  endoderm  (End)  has  been 
turned  out  instead  of  in,  thus  leaving  the  digestive  layer  of  cells  on  the 
outside  of  the  body;  Shg,  shell  gland;  V,  velum. 


230 


Heredity  and  Environment 


abnormal  environmental  conditions  in  early  stages  of  ontogeny. 
Effects  of  Food. — In  addition  to  such  monsters,  which  are  in- 
capable of  long  life,  many  peculiar  if  not  abnormal  types  of  ani- 
mals are  produced  by  the  action  of  unusual  environmental  stimuli 
during  later  stages  of  development.  Gudernatsch  found  that  if 
tadpoles  of  the  frog  were  fed  on  the  thyroid  gland  they  trans- 
formed into  minute  frogs,  scarcely  larger  than  flies,  but  if  fed 
on  thymus  gland  they  grew  to  be  large,  dark-colored  tadpoles 


FIG.  83.  YOUNG  FISH.  On  the  right  a  normal  individual  with  two  eyes ; 
on  the  left  Cyclopean  monsters  with  one  eye ;  produced  by  treatment  with 
magnesium  solutions.  (From  Stockard.) 

but  did  not  change  into  frogs;  if  fed  on  the  adrenal  gland  they 
produced  extremely  light-colored  forms.  If  canary  birds  are 
fed  on  sweet  red  pepper  they  become  red  in  color.  If  the  larvae 
of  bees  are  fed  on  "royal  jelly,"  which  is  a  bee  food  rich  in  fats, 
they  become  fertile  females  or  queens;  if  fed  on  ordinary  "bee 
bread"  they  become  infertile  females  or  workers  (Fig.  84).  There 
are  marked  structural  differences  between  the  workers  and  the 
queens  but  the  differences  in  their  habits  and  instincts  are  even 
more  striking ;  all  of  these  differences  whether  in  bodily  structure 


Influence  of  Environment  231 

B 


FIG.  84.  THE  THREE  CASTES  OF  THE  HONEY  BEE.  A,  worker  or  imper- 
fect female ;  B,  queen  or  perfect  female ;  C,  drone  or  male.  The  differ- 
ences betwen  workers  and  queens  are  produced  by  the  type  of  food  sup- 
plied to  the  larvae. 

or  in  instincts  are  determined  by  the  character  of  the  food  and 
not  by  heredity.  Innumerable  cases  of  a  similar  sort  could  be 
named  which  show  the  great  effect  of  environmental  stimuli  on 
development  but  not  upon  heredity. 

C.  FUNCTIONAL  ACTIVITY  AS  A  FACTOR  OF  DEVELOPMENT 
Another  factor  of  development  which  is  partly  intrinsic  and  part- 
ly extrinsic  is  functional  activity  or  use.  Functional  activity  is  re- 
sponse to  stimuli  which  may  be  external  or  internal  in  origin.  The 
entire  process  of  development  may  be  regarded  as  a  series  of  such 
responses  on  the  part  of  the  organism,  whether  germ  cell,  embryo 
or  adult.  The  nature  of  the  response  is  determined  by  the  nature 
and  state  of  the  organism  and  by  the  character  of  the  stimulus. 
By  the  normal,  or  usual,  series  of  stimuli  certain  parts  are  kept 
active  while  other  parts  are  kept  inactive  or  are  inhibited. 

Developmental  Movements. — Normal  development  is  depend- 
ent on  the  correlated  activity  of  many  parts  of  the  organism.  If 
in  any  part  stimuli  and  responses  are  lacking  the  development 
of  that  part  is  arrested  or  inhibited.  For  example  in  the  cleav- 
age stages  different  substances  are  sorted  and  localized  by  pro- 


232  Heredity  and  Environment 

toplasmic  movements  within  cells  and  these  substances  are  then 
isolated  by  cell  divisions  and  by  the  formation  of  partition  walls 
between  cells;  these  protoplasmic  movements  occur  in  response 
to  stimuli  and  if  these  movements  are  stopped  cleavage  and  dif- 
ferentiation are  arrested.  In  later  stages  the  infolding  of  the 
gastrula,  or  neural  tube,  or  alimentary  canal,  and  the  foldings  of 
layers  in  general,  which  play  so  important  a  part  in  development, 
are  due  to  the  movements  of  substances  within  cells  and  to  the 
movements  of  cells  in  the  layers  in  which  they  lie,  and  if  these 
movements  are  inhibited  normal  development  is  prevented. 

Nutrition  and  Development. — Another  type  of  functional  ac- 
tivity which  is  a  potent  factor  in  development  is  found  in  the 
trophic  or  nutritive  relations  which  exist  between  different  parts 
of  an  organism.  Organs  long  unused  undergo  regressive  changes 
and  may  become  rudimentary,  for  example  the  muscles  of  a  limb, 
which  has  been  paralyzed  or  placed  in  a  cast,  shrivel;  on  the 
other  hand  use  increases  the  size  and  strength  of  any  organ.  In- 
activity or  atrophy  of  one  part  usually  leads  to  imperfect  nourish- 
ment and  development  of  related  parts;  for  example,  the  optic 
nerve  atrophies  when  the  eye  is  lost,  and  muscles  atrophy  when 
the  nerves  leading  to  them  are  destroyed  or  paralyzed.  In  gen- 
eral the  normal  development  of  any  part  is  dependent  upon  its 
proper  nutrition  and  this  is  dependent  upon  the  functional  activity 
of  this  and  other  related  parts. 

Internal  Secretions;  Hormones. — Still  another  phase  of  func- 
tional activity  is  found  in  the  effects  of  certain  secretions  and 
chemical  substances  which  are  formed  by  different  glands  and 
poured  into  the  blood.  In  many  cases  the  secondary  sexual  char- 
acters which  distinguish  the  male  or  the  female  are  due  to  chemi- 
cal substances  from  the  interstitial  cells  of  the  gonads  (testes  or 
ovaries),  which  stimulate  or  exhibit  the  formation  of  these 
characters.  If  the  ovary  is  removed  from  a  young  hen  she  de- 
velops the  larger  size,  more  brilliant  plumage  and  the  pecu- 
liar comb,  wattles  and  spurs  of  the  cock.  These  secondary  sexual 


Influence  of  Environment  233 

characters  of  the  male  are  potential  in  the  female  but  are  kept 
from  developing  or  are  inhibited  by  the  activity  of  the  ovary. 
On  the  other  hand  the  castration  of  the  young  cock  does  not 
prevent  the  development  of  most  of  the  secondary  sexual  char- 
acters of  the  male.  In  the  case  of  mammals  removal  of  the  ovar- 
ies of  a  young  female  or  of  the  testes  of  a  young  male  does  not 
lead  to  the  development  of  the  secondary  sexual  characters  of 
the  other  sex,  but  both  sexes  remain  in  a  sexually  undeveloped  or 
infantile  condition,  that  is,  the  presence  of  ovaries  or  testes  serves 
as  stimulus  to  call  forth  the  development  of  the  secondary  sexual 
characters  in  mammals,  and  not  as  inhibitors  to  prevent  the  de- 
velopment of  the  secondary  sexual  characters  of  the  opposite  sex, 
as  in  the  female  fowl.  If  bits  of  the  ovary  of  a  guinea-pig  are 
inserted  under  the  skin  of  a  young  male  which  has  been  pre- 
viously castrated,  the  latter  develops  mammary  glands  similar  to 
those  of  a  normal  female;  in  short  he  is  "feminized"  by  the  stim- 
ulus of  substances  from  the  ovary. 

Another  gland  whose  secretions  exercise  a  profound  influence 
on  development  is  the  thyroid,  which  is  found  in  the  neck  near  the 
"Adam's  apple."  If  the  secretion  of  this  gland  is  over-abundant 
it  causes  rapid  heart-beat,  higher  temperature,  increased  activity, 
and  in  general  a  higher  rate  of  metabolism,  and  if  the  gland  be- 
comes much  enlarged  it  gives  rise  in  addition  to  goitre  and  pro- 
truding eyeballs.  Excess  of  the  thyroid  secretion  hastens  certain 
processes  of  development;  young  tadpoles  that  have  been  fed  on 
thyroid  gland  or  its  extract  quickly  transform  into  frogs  which 
are  sometimes  "no  larger  than  flies"  (Gudernatsch).  On  the  other 
hand  if  the  thyroid  secretion  is  deficient  the  rate  of  metabolism  is 
decreased,  body  temperature  is  lowered,  muscular  movements  be- 
come slow  and  awkward,  the  skin  may  become  swollen  with  mu- 
cous (myxedema)  and  the  mind  becomes  dull  or  stupid.  Thyroid 
deficiency  in  a  young  child  prevents  the  normal  development  of 
body  and  mind  and  in  extreme  cases  ca.uses  that  peculiar  type  of 
idiotic  dwarf  known  as  "cretin."  If  thyroid  extract  is  admin- 


234  Heredity  and  Environment 

istered  early  enough  in  such  cases  the  most  wonderful  improve- 
ment is  brought  about;  the  body  frequently  assumes  normal 
proportions  and  features  and  the  mind  becomes  bright  and  active. 

Another  gland  of  internal  secretion  which  plays  a  great  part 
in  the  development  of  body  and  mind  is  the  pituitary,  or  hypo- 
physis, which  lies  on  the  ventral  side  of  the  brain ;.  it  consists  of 
two  parts,  an  anterior  portion  derived  embryologically  from  the 
roof  of  the  mouth,  and  a  posterior  portion  which  came  from  the 
floor  of  the  brain.  Excessive  secretion  of  the  anterior  portion 
stimulates  the  growth  of  the  bones  and  if  this  occurs  in  early  life 
it  leads  to  a  great  lengthening  of  the  bones,  especially  of  the  arms 
and  legs,  and  in  later  life  the  bones  of  the  hands,  feet  and  face 
becomes  enlarged  so  that  the  subject  becomes  a  giant,  though  often 
with  weakened  body  and  mind ;  on  the  other  hand  if  there  is  de- 
ficiency of  secretion  from  the  anterior  pituitary  there  is  a  general 
dwarfing  of  the  body.  The  secretion  of  the  posterior  pituitary 
acts  as  a  stimulant  to  nerve  cells,  involuntary  muscles  and  to  the 
sex  glands ;  deficiency  of  this  secretion  causes  persistent  "infantil- 
ism" or  sexual  immaturity. 

Another  pair  of  glands  of  internal  secretion  which  exercise  a 
profound  influence  on  both  the  structures  and  functions  of  the 
body  are  the  adrenals,  or  suprarenals,  located  just  above  the  kid- 
neys. These  glands  also  are  composed  of  two  parts  which  produce 
secretions  differing  in  their  physiological  action, — namely,  a  central 
part  or  medulla  and  a  superficial  part  or  cortex.  The  secretion 
of  the  medulla  is  known  as  adrenin  or  epinephrin  and  when  intro- 
duced into  the  blood  it  strengthens  the  heart  beat  and  causes  the 
blood  to  be  driven  from  the  abdominal  viscera  to  the  skeletal  mus- 
cles, heart,  lungs  and  brain.  It  also  causes  the  liver  to  set  free, 
into  the  blood,  sugar,  which  is  "the  optimum  source  of  muscular 
energy" ;  it  quickly  restores  fatigued  muscle  and  increases  the 
coagulability  of  the  blood  Cannon  lias  shown  that  in  emotional 
excitement,  such  as  pain,  hunger,  fear,  or  rage,  there  is  increased 
secretion  of  adrenin  into  the  blood,  and  he  has  called  attention  to 


Influence  of  Environment  235 

the  remarkably  adaptive  character  of  all  these  reactions.  Prob- 
ably the  ability  to  "nerve  oneself"  to  meet  emergencies  successfully 
depends  upon  this  secretion.  In  cases  of  adrenin-insufficiency 
there  is  a  lowering  of  blood  pressure,  lack  of  muscular  tone  and 
"loss  of  nerve,"  such  as  occur  in  neurasthenia,  "shell  shock,"  etc. 
On  the  other  hand  the  secretion  of  the  cortex  is  said  to  stimulate 
the  development  of  the  sex  glands,  and  to  hasten  sexual  maturity. 
Deficiency  of  this  secretion,  as  in  Addison's  disease,  causes  a 
bronzing  of  the  skin  and  it  has  been  suggested  by  Keith  that  the 
skin  colors  of  the  darker  races  of  mankind  are  due  to  a  relative 
deficiency  of  this  secretion,  while  "we  Europeans  owe  the  fairness 
of  our  skins  to  some  particular  virtue  resident  in  the  adrenal 
bodies."  Keith  has  further  suggested  that  many  other  racial  char- 
acteristics such  as  shape  and  size  of  head,  face,  nose,  eyes,  teeth 
and  lips,  length  of  arms  and  legs,  character  and  abundance  of  hair 
on  various  parts  of  the  body,  etc.,  may  be  correlated  with  the 
relative  activity  of  the  several  glands  of  internal  secretion.  He 
believes  that  in  general  the  white  race  possesses  more  of  the 
internal  secretions  of  gonad,  thyroid,  pituitary  and  adrenal  than 
do  the  other  races.  While  these  suggestions  are  highly  speculative, 
they  do  serve  to  emphasize  the  great  importance  of  these  secre- 
tions on  the  development  of  body,  mind,  and  personality. 

Since  racial  characters  are  inherited  it  is  necessary  to  assume 
that  in  some  way  the  chromosomes  of  the  oosperm  influence  the 
development  of  the  glands  of  internal  secretion,  and  through 
these,  variously  bodily  and  mental  characteristics.  The  influence 
of  the  internal  secretions  on  development  does  not  disprove  the 
importance  of  heredity,  but  rather  it  points  out  a  mechanism 
through  which  heredity  controls  development. 

Correlative-Differentiation  and  Self -Differentiation. — Many 
cases  are  known  in  which  the  development  of  a  part  is  dependent 
upon  the  presence  of  another  part;  this  is  technically  known  as 
"correlative  differentiation."  Thus  it  has  been  found  that  the  lens 
of  the  eye  will  develop  from  any  portion  of  the  ectoderm,  or  outer 


236  Heredity  and  Environment 

layer  of  the  skin,  if  only  the  primitive  retina,  or  optic  cup,  is 
brought  near  to  this  layer;  if  the  optic  cup  is  transplanted  from 
the  head  to  the  thorax  or  abdomen  a  lens  will  form  wherever  the 
cup  comes  in  contact  with  the  ectoderm.  If  an  embryonic  limb  is 
transplanted  from  its  normal  position  to  the  middle  of  the  back  or 
belly,  it  will  develop,  and  nerves  and  blood  vessels  will  grow  into 
It  which  would  have  had  very  different  positions  and  distributions 
if  the  limb  had  not  been  there.  If  one  of  the  first  four  cleavage 
cells  is  separated  from  the  others  it  may  develop  into  an  entire  ani- 
mal though  it  would  have  formed  only  a  quarter  of  an  animal  if 
it  had  remained  in  contact  with  the  other  three-quarters  of  the 
egg.  All  such  cases  are  known  as  "correlative  differentiation," 
implying  that  differentiation  is  dependent  upon  the  stimuli  which 
come  from  surrounding  parts.  On  the  other  hand  if  the  differen- 
tiation has  already  begun  before  the  relation  of  a  part  to  surround- 
ing parts  has  been  changed,  it  may  continue  to  differentiate  as  if 
no  change  of  position  or  relation  had  taken  place.  Thus  if  a  right 
limb  is  transplanted  to  the  left  side  of  the  body  after  it  has  begun 
to  differentiate  it  remains  a  right  limb  and  is  not  modified  by  its 
new  relations  (Harrison)  ;  if  the  cleavage  cells  are  already  dif- 
ferentiated in  the  four-celled  stage,  each  cell  when  separated  from 
the  others  will  give  rise  to  only  one-quarter  of  an  animal.  In 
short  the  organ  or  cell  is  already  set,  or  fixed,  or  differentiated 
to  such  an  extent  that  it  can  not  change  its  fate  even  though 
its  environment  should  change.  Such  cases  are  known  as  "self- 
differentiation." 

Many  students  of  the  physiology  of  development  have  been 
led  to  the  view  that  the  fundamental  causes  of  development  are  to 
be  found  not  in  the  egg  cell  itself  but  in  environmental  stimuli 
and  in  the  interaction  of  the  various  parts.  Driesch  in  particular 
regards  the  egg,  or  any  cleavage  cell,  as  an  "harmonic  equipoten- 
tial  system,"  that  is,  any  part  is  capable  of  any  fate  and  its  actual 
fate  is  determined  by  its  relation  to  other  parts;  in  the  striking 
phrase  of  Driesch,  "The  fate  of  a  part  is  a  function  of  its  posi- 


Influence  of  Environment  237 

tion."  We  now  know  that  this  expresses  only  a  fraction  of  the 
truth.  The  fate  of  a  part  is  primarily  determined  by  its  proto- 
plasmic organization  and  only  secondarily  by  its  position. 

These  are  only  a  few  illustrations  of  the  many  kinds  of  abnor- 
mal development  which  may  be  caused  by  changed  environment  or 
by  unusual  functional  activities.  At  all  stages  of  ontogeny  the 
course  of  development  may  be  altered  by  extrinsic  stimuli  but 
earlier  stages  may  be  more  profoundly  influenced  than  later  ones. 

D.  INHERITANCE  OR  NON-INHERITANCE  OF 
ACQUIRED  CHARACTERS 

Few  questions  in  biology  have  been  discussed  so  fully  and  so 
fruitlessly  as  this.  It  is  a  problem  of  the  greatest  interest  not 
only  to  students  of  biology  but  also  to  sociologists,  educators  and 
philanthropists  and  yet  it  is  still  to  a  certain  extent  an  unsolved 
problem. 

Opinions  of  Lamarck  and  Darwin. — It  is  well  known  that  La- 
marck taught  that  characters  due  to  desire  or  need,  use  or  disuse, 
and  to  changed  environment  or  conditions  of  life  were  inherited 
and  thus  brought  about  progressive  evolution.  Long  ago  desire 
or  need  was  repudiated  as  a  factor  of  evolution.  Lowell  satir- 
ized it  in  his  Biglow  Papers  in  these  words : 

"Some  filosifers  think  that  a  fakkilty's  granted 

The  minnit  it's  felt  to  be  thoroughly  wanted, 

***** 

That  the  fears  of  a  monkey  whose  holt  chanced  to  fail 
Drawed  the  vertibry  out  to  a  prehensile  tail." 

Darwin  wrote  to  Hooker,  "Heaven  forfend  me  from  Lamarck's 
nonsense  of  adaptation  from  the  slow  willing  of  animals" ; 
but  although  he  repudiated  this  feature  of  Lamarckism  he  held 
that  characters  due  to  use  or  disuse  and  to  changed  conditions  of 
life  might  be  inherited  and  he  proposed  his  hypothesis  of  pan- 
genesis  in  order  to  explain  the  process  of  the  transmission  of 
such  characters  to  the  germ  cells. 


238  Heredity  and  Environment 

Weismann's  Theories. — Weismann  introduced  a  new  era  in 
biology  by  denying  the  inheritance  of  all  kinds  of  acquired  char- 
acters, and  by  challenging  the  world  to  produce  evidence  that 
would  stand  a  rigorous  analysis.  But  Weismann's  greatest  ser- 
vice lay  in  his  constructive  theories  rather  than  in  destructive 
criticism;  he  forever  disposed  of  theories  of  pangenesis  and  the 
like  by  showing  that  the  germ  cells  are  not  built  up  by  contribu- 
tions from  the  body  and  that  characters  are  not  transmitted  from 
generation  to  generation;  but  on  the  other  hand  that  there  is 
transmitted  a  germplasm  which  is  relatively  independent  of  the 
body  and  which  te  relatively  very  stable  in  organization.  This 
epoch-making  theory  of  Weismann's  has  naturally  undergone 
some  changes,  as  the  result  of  new  discoveries.  It  is  no  longer 
believed  that  the  germplasm  is  really  independent  of  the  body, 
nor  that  it  is  absolutely  stable,  as  Weismann  at  one  time  held. 
There  is  no  doubt  that  the  germ  cells  and  the  germplasm  are 
physiologically  related  to  other  cells  and  to  other  plasms,  and 
similarly  there  is  no  doubt  that  the  germplasm  although  very 
stable  can  and  does  change  its  constitution  under  some  rare 
conditions.  But  in  the  main  the  germplasm  theory  is  accepted  by 
the  great  majority  of  biologists  to-day,  and  recent  work  in 
genetics  and  cytology  has  brought  many  confirmations  of  this 
theory. 

Distinction  between  Hereditary  and  Acquired  Characters. — As 
long  as  it  was  believed  that  the  developed  characters  of  an  or- 
ganism could  be  transmitted  as  such  to  its  descendants  it  was  cus- 
tomary to  speak  of  developed  characters  as  hereditary  or  ac- 
quired and  to  talk  of  the  inheritance  or  non-inheritance  of  acquired 
characters.  This  distinction  is  not  a  logical  one  for  all  developed 
characters  are  invariably  the  result  of  the  responses  of  the  ger- 
minal organization  to  environmental  stimuli;  and  of  course  no 
developed  character  can  be  purely  hereditary  or  purely  environ- 
mental. But  when  a  given  character  arises  in  many  individuals 
of  the  same  biotype  under  different  environmental  conditions  it 


Influence  of  Environment  239 

is  probable  that  heredity,  which  is  the  constant  factor  in  this 
case,  is  also  the  determining  factor  for  that  character.  On  the 
other  hand  if  a  character  develops  in  response  to  peculiar  stimuli 
and  does  not  appear  in  other  individuals  of  the  same  biotype  in 
which  such  stimuli  are  lacking  it  is  said  to  be  an  environmental 
or  acquired  character.  In  fine,  inherited  characters  are  those 
whose  distinctive  or  differential  causes  are  in  the  germ  cells, 
while  acquired  characters  are  those  whose  differential  causes  are 
environmental. 

Statement  of  Problem. — Briefly  stated  the  question  of  the  in- 
heritance of  acquired  characters  is  this:  Can  the  differential 
cause  of  a  character  be  shifted  from  the  environment  to  the  germ- 
plasm?  Can  peculiarities  of  the  environment  which  influence 
the  development  of  somatic  characters  so  affect  the  germ  cells 
that  they  will  produce  these  somatic  characters  in  the  absence  of 
the  peculiar  environment  ?  Can  the  characteristics  of  a  developed 
organism  enter  into  its  germ  cells  and  be  born  again  in  the  next 
generation?  Considering  the  fact  that  germ  cells  are  cells  and 
contain  no  adult  characteristics,  it  seems  very  improbable  that  any 
peculiarity  of  environment  whether  of  nutrition,  use,  disuse  or 
injury,  which  brings  about  certain  peculiarities  of  developed  char- 
acters in  the  adult,  could  so  change  the  structure  of  the  germ  cells 
as  to  cause  them  to  produce  this  same  character  in  subsequent 
generations  in  the  absence  of  its  extrinsic  cause.  How,  for  ex- 
ample, could  defective  nutrition,  which  leads  to  the  production 
of  rickets,*  affect  the  germ  cells,  which  contain  no  bones,  so  as  to 
produce  rickets  in  subsequent  generations,  although  well  nour- 
ished? Or  how  can  over-exertion,  leading  to  hypertrophy  of  the 
heart,  so  affect  the  germ  cells  that  they,  in  turn,  would  produce 
hypertrophied  hearts  in  the  absence  of  over-exertion,  seeing  that 
germ  cells  have  no  hearts?  Or  how  could  the  loss  or  injury  of 

*  It  has  recently  been  shown  that  rickets  may  also  be  produced  by  ab- 
sence of  sunlight  or  other  radiant  energy.  It  may  be  prevented  by  ex- 
posure to  sunlight,  if  taken  in  time. 


240  Heredity  and  Environment 

eyes  or  teeth  or  legs  lead  to  the  absence  or  weakened  development 
of  these  organs  in  future  generations,  seeing  that  inheritance  must 
be  through  germ  cells  which  possess  none  of  these  structures  ? 

Lack  of  Evidence  for  Inheritance  of  Acquired  Characters. — 
But,  apart  from  these  general  objections  to  the  doctrine  of  the  in- 
heritance of  acquired  characters,  there  are  many  special  difficul- 
ties. There  is  little  or  no  conclusive  and  satisfactory  evidence  in 
favor  of  such  inheritance.  Almost  all  the  evidence  adduced  serves 
to  show  only  that  characters  are  acquired,  not  that  they  are 
inherited. 

It  is  a  matter  of  common  observation  that  mutilations  are  not 
inherited;  wooden  legs  do  not  run  in  families,  although  wooden 
heads  do.  The  evidence  for  the  inheritance  of  peculiarities  due 
to  use  or  disuse  is  wholly  inconclusive;  for  example,  did  the 
giraffe  get  his  long  neck  because  he  browsed  on  trees,  or  does  he 
browse  on  trees  because  he  has  by  inheritance  a  long  neck?  Did 
attempts  to  fly  lead  to  the  development  of  wings  in  birds,  or  do 
birds  fly  because  heredity  has  given  them  wings?  Did  life  in 
caves  make  cave  animals  blind,  or  did  blind  animals  resort  to 
caves  because  the  struggle  for  existence  there  was  less  severe 
for  them?  The  evidence  is  in  favor  of  the  second  of  each  of 
these  alternatives  rather  than  of  the  first. 

There  still  remains  the  question  of  the  inheritance  of  certain 
characters  due  to  environment,  though  here  also  the  most  clear- 
cut  evidence  is  against  this  proposition.  That  unusual  conditions 
of  food,  temperature,  moisture,  etc.,  may  affect  the  germ  cells  so 
as  to  produce  general  and  indefinite  variations  in  offspring  is 
probable,  but  this  is  a  very  different  thing  from  the  inheritance 
of  acquired  characters.  The  germ  cells  being  a  part  of  the  paren- 
tal organism  may  be  modified  by  such  changes  in  the  environment 
as  affect  the  body  as  a  whole,  they  may  be  well  nourished  or 
starved,  they  may  be  modified  by  changed  conditions  of  gravity, 
salinity,  pressure,  temperature,  etc.,  and  these  modifications  of  the 
germ  cells  probably  lead  to  certain  general  modifications  of  the 


Influence  of  Environment 


241 


adult,  which  may  be  larger  or  smaller,  stronger  or  weaker,  accord- 
ing as  the  germ  is  well  or  poorly  nourished,  but  it  is  incredible  that 
the  environment  which  produces  rickets,  or  hypertrophied  heart, 
or  loss  of  sight  in  one  generation  should  modify  the  germ  cells 
in  such  a  peculiar  and  definite  way  that  they  should  give  rise  in 
the  next  generation  to  these  particular  peculiarities,  in  the  absence 
of  the  extrinsic  cause  which  'first  produced  them.  The  inheri- 


FIG.  85.  GRAFTED  FROG  EMBRYOS,  anterior  part,  Rana  sylvatica,  posterior 
part,  R.  palustris.  In  later  stages,  and  even  in  the  adult  condition,  the  two 
parts  preserve  their  peculiarities.  (From  Harrison.) 

tance  of  acquired  characters  is  incredible,  because  the  egg  is  a 
cell  and  not  an  adult  organism ;  and  in  this  case  there  is  no  suffi- 
cient evidence  that  the  thing  which  is  incredible  really  does  hap- 
pen. 


242  Heredity  and  Environment 

No  Inherited  Influence  of  Stock  on  Graft. — If  specific  changes 
of  environment  produced  specific  changes  in  heredity  we  should 
expect  to  find  that  where  different  plants  or  animals  are  grafted 
together  each  would  modify  more  or  less  the  hereditary  consti- 
tution of  the  other.  But  this  does  not  occur.  Everybody  knows 
that  when  a  branch  of  a  particular  kind  of  fruit  tree  is  grafted 
upon  a  tree  of  a  different  variety  the  quality  of  the  fruit  borne  by 
that  branch  is  not  altered  by  its  close  union  with  the  new  stock. 
The  same  is  true  of  all  forms  of  animal  grafts.  Harrison  cut  in 
two  young  tadpoles  of  two  species  of  frog,  Rcma  sylvatica  and 
Rana  palustris,  and  spliced  the  anterior  half  of  one  to  the  posterior 
half  of  the  other.  These  frogs  and  their  tadpoles  differ  in  color 
as  well  as  in  other  respects,  R.  sylvatica  being  more  deeply  pig- 
mented  than  R.  palustris.  In  the  grafted  tadpoles  each  half 
preserved  its  own  peculiarities  even  up  to  the  adult  condition 
(Fig.  85). 

A  still  more  striking  case  of  the  persistence  of  heredity  in  spite 
of  environmental  changes  is  found  in  experiments  in  which  the 
ovaries  are  removed  from  one  variety  of  animal  and  transplanted 
to  another  variety.  Guthrie  made  such  transplantations  in  the  case 
of  fowls  and  concluded  that  there  was  some  influence  of  the  fos- 
ter mother  upon  the  transplanted  ovary,  but  Davenport,  who  re- 
peated his  experiments,  was  unable  to  confirm  his  results ;  on  the 
contrary  he  frequently  found  that  the  engrafted  ovary  degenerated 
and  that  the  excised  ovary  regenerated.  Finally  Castle  and 
Phillips  furnished  the  most  conclusive  demonstration  that  the 
hereditary  characteristics  of  the  transplanted  ova  are  in  no  wise 
changed  by  the  foster  mother.  They  removed  the  ovary  from  a 
pure  black  guinea-pig  and  put  it  in  the  place  of  the  ovary  of  a 
pure  white  animal.  After  recovery  from  the  operation  this 
white  female  with  the  "black"  ovary  was  bred  to  a  pure  white 
male  (Fig.  86).  Three  litters  of  offspring  from  these  parents 
were  all  black  as  shown  in  Figure  87.  Although  both  parents 
were  pure  white  all  the  offspring  of  the  F:  generation  were  black 


Influence  of  Environment  243 

because  they  came  from  ' 'black"  eggs  and  black  is  dominant  over 
white.  The  fact  that  these  "black"  eggs  developed  in  the  body  of 
a  white  female  did  not  in  the  least  change  their  hereditary  con- 
stitution. 

Dominants  and  Recessives  Remain  Pure. — A  still  more  inti- 
mate union  takes  place  when  the  dominant  and  recessive  char- 
acters come  together  in  any  zygote.  These  characters,  or  rather 
the  factors  which  determine  them,  may  be  intimately  associated 
in  every  cell  of  the  organism  throughout  an  entire  generation 
and  yet  we  get  a  clean  separation  of  these  characters  in  the  next 
generation ;  neither  the  dominant  nor  the  recessive  character  has 
been  at  all  modified  by  its  mostantimate  association  with  the  other. 

Climatic  Effects  Not  Inherited. — A  striking  instance  of  the 
purely  temporary  effect  of  the  environment  and  of  the  long 
persistence  of  hereditary  constitution  amidst  new  environmental 
conditions,  which  have  greatly  changed  the  appearance  of  the 
developed  organisms,  is  found  in  the  case  of  alpine  plants.  Nageli 
says  that  such  plants,  which  have  preserved  the  characters  of  high 
mountain  plants  since  the  ice  age,  lose  these  characters  perfectly 
during  their  first  summer  in  the  lowlands. 

Summary. — If  acquired  characters  were  really  inherited  we 
should  expect  to  find  many  positive  evidences  of  this  instead  of  a 
few  sporadic  and  doubtful  cases.  In  particular  why  do  we  not 
find  in  plant  or  animal  grafting  that  the  influence  of  the  stock 
changes  the  hereditary  potencies  of  the  graft?  Why  do  we  not 
find  that  transplanted  ovaries  show  the  influence  of  the  foster 
mother  as  Guthrie  supposed — a  thing  which  has  been  disproved 
by  Davenport  and  Castle  (Figs.  86  and  87)  ?  Why  do  dominant 
and  recessive  characters  remain  pure,  even  after  their  intimate 
union  in  a  hybrid,  so  that  pure  dominants  and  pure  recessives  may 
be  obtained  in  subsequent  generations  from  this  mixture?  Why 
does  every  child  have  to  learn  anew  what  his  parents  learned  so 
laboriously  before  him?  Even  the  strongest  defenders  of  the 
inheritance  of  acquired  characters  are  constrained  to  admit  that 
it  occurs  only  sporadically  and  exceptionally. 


244 


Heredity  and  Environment 


FIG.  86.  EFFECT  OF  TRANSPLANTING  OVARIES  IN  GUINEA-PIGS.  Above, 
young  black  female;  in  the  middle,  mature  white  female;  below,  mature 
white  male.  The  white  female's  ovary  was  removed  and  in  its  place  was 
put  the  ovary  from  the  black  female.  The  white  female  (with  "black" 
ovary)  was  then  bred  to  the  white  male.  (From  Castle.) 


Influence  of  Environment 


245 


FIG.  87.  RESULTS  OF  CROSS  DESCRIBED  IN  THE  PRECEDING  FIGURE.  All 
the  offspring  are  black,  though  both  parents  are  white,  because  the  white 
female  contains  only  "black"  eggs  and  black  is  dominant  over  white. 


246  Heredity  and  Environment 

Neo-Lamarckism. — Many  modifications  of  the  Lamarckian  hy- 
pothesis of  the  inheritance  of  acquired  characters  have  been  pro- 
posed in  recent  years.  Foremost  among  these  are  the  "mneme" 
theory  of  Semon  and  the  "centro-epigenesis"  theory  of  Rignano. 
To  Semon  as  to  many  other  biologists  the  apparent  resemblance 
between  memory  and  heredity  has  seemed  significant,  and  this 
furnishes  the  basis  of  his  theory.  Semon  holds  that  every  condi- 
tion of  life,  every  functional  activity  of  an  organism  leaves  a 
permanent  record  of  itself  in  what  he  calls  an  "engramme."  If 
these  conditions  or  activities  are  long  continued  their  engrammes 
are  heaped  up  and  affect  heredity.  Semon  does  not  ask  if  "ac- 
quired characters"  are  inherited,  but  rather  "Are  the  hereditary 
potencies  of  the  germ  cells  altered  by  stimuli  acting  on  the  paren- 
tal body  ?"  This  is  a  very  different  thing  from  the  inheritance  of 
a  particular  acquired  character,  and  there  is  some  evidence  that 
such  stimuli  may  in  rare  instances  produce  changes  in  the  heredi- 
tary constitution  of  the  germplasm  though  these  evidences  are 
by  no  means  conclusive. 

Temporary  Effects  of  Environment;  "Induction." — On  the 
other  hand  certain  changes  may  be  produced  in  germ  cells  or 
embryos  which  last  for  only  a  generation  or  two  and  then  dis- 
appear. It  is  well  known  that  plants  grown  in  poor  soil  are 
smaller  and  generally  produce  smaller  seeds  than  those  grown  in 
good  soil,  and  deVries,  Baur  and  Harris  find  that  such  seeds  pro- 
duce smaller  plants  having  smaller  seeds  than  do  seeds  of  normal 
size.  This  is  an  after-effect  of  poor  nutrition  which  changes  the 
amount  of  food  material  in  the  seeds  and  through  this  the  size  of 
the  plant  which  develops  from  the  seed,  but  it  does  not  change  the 
hereditary  constitution.  Woltereck  found  that  in  Daphnia  there  is 
an  after-effect  of  cold  lasting  for  one  or  two  generations,  and  this 
he  calls  "induction"  when  the  effect  lasts  for  one  generation,  or 
"pre-induction"  when  it  lasts  for  two  or  three  generations.  Whit- 
ney found  that  rotifers  poisoned  with  alcohol  were  weaker  in 
resistance  to  copper  salts  and  were  less  fertile  than  others,  and 


Influence  of  Environment  247 

when  brought  back  to  normal  conditions  the  first  generation  was 
weak  but  the  second  was  normal.  On  -the  other  hand  Stockard 
finds  that  the  injurious  effects  of  alcohol  on  guinea  pigs  persist 
through  two  or  more  generations.  In  man  alcohol  may  have  an 
"induction"  effect  on  offspring,  but  fortunately  it  does  not  seem 
to  alter  hereditary  constitution.  Probably  of  a  similar  character 
are  Stunner's  results ;  he  found  that  mice  raised  in  the  cold  have 
shorter  tails  than  those  raised  at  higher  temperatures  and  this 
modified  character  appears  in  the  next  generation.  If  this  is  an 
after-effect  or  ''induction"  it  should  disappear  in  the  following 
generations. 

Kammerer  found  that  salamanders  with  black  and  yellow  spots 
when  reared  on  yellow  soil  gradually  ^lose  their  black  color  becom- 
ing more  yellow,  and  their  young  continue  to  grow  more  yellow 
until  finally  almost  all  black  may  disappear.  The  offspring  of 
such  salamanders  are  said  to  be  more  yellow  than  normal;  but 
this  work  has  been  called  in  question  and  needs  confirmation. 
Even  if  confirmed  the  result  may  be  an  after-effect  or  "induction" 
which  would  soon  disappear  under  usual  conditions,  and  as  yet 
there  is  no  conclusive  evidence  that  it  is  really  inherited. 

Probably  such  cases  are  not  instances  of  true  inheritance ;  they 
do  not  signify  a  change  in  the  hereditary  constitution  but  an  influ- 
ence on  the  germ  cells  of  a  nutritive  or  chemical  sort  comparable 
with  what  takes  place  when  fat  stains  are  fed  to  animals ;  the  eggs 
of  such  animals  are  stained  and  the  young  which  develop  from 
such  eggs  are  also  stained,  though  the  germinal  constitution  re- 
mains unchanged.  The  very  fact  that  the  changed  condition  is 
reversible  and  that  it  disappears  within  a  short  time  is  evidence 
that  it  is  not  really  inherited. 

One  of  the  most  interesting  and  convincing  cases  of  the  inheri- 
tance of  an  experimentally  induced  character  has  been  reported  by 
Guyer  and  Smith*  with  respect  to  certain  eye  defects  in  rabbits. 

*  Guyer,  M.  F.,  and  Smith,  E.  A.  Studies  on  Cytolysins.  II.  Trans- 
missions of  Induced  Eye  Defects.  Jour.  Exp.  Zool.  31,  Aug.  1920. 


248  Heredity  and  Environment 

They  injected  the  pulped  lenses  of  rabbits  into  fowls  and,  after 
the  fowls  had  become  sensitized  to  this  foreign  protein  by  the 
formation  of  anti-lens  substances,  their  serum  was  injected  into 
pregnant  female  rabbits.  The  effects  on  the  injected  rabbits  were 
severe  and  many  of  them  died,  but  there  was  no  evidence  that 
their  eyes  or  lenses  suffered  injury ;  furthermore  there  was  no 
evidence  of  any  specific  injury  to  their  ovarian  eggs,  since  in  sub- 
sequent breeding  they  produced  no  young  with  eye  defects.  On 
the  other  hand,  some  of  the  embryos  in  utero  did  suffer  specific 
injury ;  some  were  born  with  opaque  lenses ;  sometimes  their  lenses 
were  reduced  in  size,  and  when  the  lens  was  small  the  whole  eye 
was  usually  small;  sometimes  the  eyeball  had  collapsed  leaving 
no  trace  of  pupil  or  iris;  finally  these  degenerative  changes  fre- 
quently increased  and  progressed  after  birth. 

All  of  these  defects  might  be  explained  by  the  direct  action  of 
the  anti-lens  substances  of  the  fowl's  serum  upon  the  developing 
eyes  of  the  embryos,  which  would  be  merely  another  case  of  "in- 
duction"; but  the  thing  which  cannot  be  explained  so  easily  is 
the  fact  that  these  eye  defects  are  inherited  for  at  least  five 
generations  and  that  they  do  not  gradually  disappear  but  become 
more  pronounced  in  successive  generations.  Furthermore  it  is 
not  possible  to  assume  that  the  lens  anti-bodies  are  transmitted 
only  through  the  cytoplasm  of  the  egg,  as  plastids  are  in  plants  or 
fat-stains  in  animals,  for  the  induced  eye  defects  are  inherited 
through  the  male  as  well  as  through  the  female  line.  The  authors 
suggest  the  possibility  "that  the  degenerating  eyes  are  themselves 
directly  or  indirectly  originating  anti-bodies  or  other  chemical 
substances  in  the  blood-serum  of  their  bearers  which  in  turn  affect 
the  germ  cells." 

Whatever  the  mechanism  of  this  inheritance  may  be  it  does 
seem  that  in  this  case  there  is  specific  inheritance  of  an  induced 
injury,  and  it  will  no  doubt  give  much  comfort  to  those  who  believe 
in  the  "inheritance  of  acquired  characters."  But,  however  these 
results  may  be  interpreted,  it  is  evident  that  the  old  doctrine  of  the 


Influence  of  Environment  249 

inheritance  of  acquired  characters  due  to  use,  disuse,  or  external 
environment  and  the  crude  mechanisms  proposed  for  such  inheri- 
tance are  not  confirmed  by  these  experiments.  It  remains  to  be 
seen  whether  the  more  subtile"  influences  of  the  internal  environ- 
ment, such  as  hormones,  anti-bodies  and  enzymes  may  affect  the 
germplasm  in  a  specific  manner,  and  thus  modify  inheritance. 

In  conclusion :  ( i )  Developed  characters,  whether  "acquired" 
or  not,  are  never  transmitted  by  heredity,  and  the  hereditary  con- 
stitution of  the  germ  is  not  changed  by  changes  in  such  charac- 
ters. (2)  Possibly  environmental  stimuli  acting  upon  germ  cells 
at  an  early  stage  in  their  development  may  rarely  cause  changes 
in  their  hereditary  constitution,  but  changes  produced  in  somatic 
cells  do  not  usually,  if  ever,  cause  corresponding  changes  in  the 
hereditary  constitution  of  the  germ  cells.  (3)  Germ  cells  like 
somatic  cells  may  undergo  modifications  which  are  not  hereditary ; 
if  starved  they  may  produce  stunted  individuals  and  this  effect 
may  last  for  two  or  three  generations ;  they  may  be  stained  with 
fat  stains  and  the  generation  to  which  they  give  rise  be  similarly 
stained ;  they  may  be  poisoned  with  alcohol  or  modified  by  unusual 
temperature  and  such  influence  may  be  carried  over  to  the  next 
generation  without  becoming  hereditary.  All  such  cases  are 
known  as  "induction"  and  many  instances  of  the  supposed  in- 
heritance of  acquired  characters  come  under  this  category.  (4) 
Environment  may  profoundly  modify  individual  development  but 
it  does  not  generally  modify  heredity. 

E.  APPLICATIONS  TO  HUMAN  DEVELOPMENT:  EUTHENICS 

Man's  Larger  Environment. — Man's  environment  is  more  ex- 
tensive than  that  of  any  other  animal,'  and  its  influence  on  his  de- 
velopment is  correspondingly  greater.  In  addition  to  chemical 
and  physical  stimuli  which  are  potent  factors  of  development  in 
the  case  of  all  organisms,  man  lives  in  a  world  of  psychical,  so- 
cial and  moral  stimuli  which  exert  a  profound  influence  on  him. 
He  is  stimulated  not  merely  by  present  environment  but  also  by 


250  Heredity  and  Environment 

memories  of  past  experiences  and  anticipations  of  future  ones. 
Through  intelligence  and  social  cooperation  he  is  able  to  control 
environment  for  particular  ends,  in  a  manner  quite  impossible 
to  other  organisms.  On  the  other  hand  heredity  as  a  factor  of 
development  is  no  more  powerful  in  the  case  of  mari  than  in  any 
other  organism.  Consequently  the  relative  importance  of  hered- 
ity and  environment  is  not  the  same  in  the  development  of  an 
intelligent  and  social  being,  like  man  of  the  present  age,  as  it  is 
in  lower  organisms.  For  man  and  for  every  other  living  crea- 
ture heredity  fixes  the  possibilities  of  development,  it  "sets  bounds 
about  us  which  we  cannot  pass";  but  the  more  complex  those 
possibilities  become  the  more  complex  must  be  the  environment 
which  calls  them  forth  and  the  more  varied  become  the  results  of 
development  under  altered  conditions  of  life. 

Capacity  for  Training  and  Education. — Functional  activity  also 
plays  a  larger  part  in  man's  development  than  in  that  of  any  other 
animal,  owing  to  the  longer  period  of  his  development  and  to  the 
more  extensive  and  varied  training  which  he  is  capable  of  under- 
going. It  is  a  notable  fact  that  the  period  of  immaturity  in  man 
is  longer  than  in  any  other  animal,  and  it  is  during  this  formative 
period  that  environment  and  education  have  their  greatest  influ- 
ence. Other  animals  develop  much  more  rapidly  than  man  but 
that  development  sooner  comes  to  an  end.  The  children  of  lower 
races  of  mankind  develop  more  rapidly  than  those  of  higher  races 
but  in  such  cases  they  also  cease  to  develop  at  an  earlier  age. 
The  prolongation  of  the  period  of  infancy  and  of  immaturity  in 
the  human  race  greatly  increases  the  importance  of  environment 
and  training  as  factors  of  development. 

The  possible  training  of  human  faculties  is  also  more  varied 
and  extensive  than  in  other  animals,  not  only  because  those  facul- 
ties are  more  numerous  but  also  because  they  are  more  plastic 
and  are  capable  of  higher  development,  that  is,  are  more  edu- 
cable.  Human  faculties  are  functions  and  methods  of  reaction, 
which  are  dependent  in  part  upon  the  bodily  mechanism  and  in 


Influence  of  Environment  251 

part  upon  environment  and  training.  Habits  are  the  usual  meth- 
ods of  responding  to  stimuli,  and  they  may  be  classified  as  in- 
herent or  acquired.  The  former  are  instincts  or  reflexes  and  are 
expressions  of  hereditary  constitution;  the  latter  are  in  a  sense 
forced  upon  organisms  by  environmental  conditions.  All  educa- 
tion is  habit  formation,  and  good  education  like  good  environment 
is  such  experience  as  leads  to  the  formation  of  good  bodily,  in- 
tellectual, social  and  moral  habits ;  it  consists  in  placing  the  in- 
dividual in  such  an  environment  and  bringing  such  stimuli  to 
bear  upon  him  as  to  call  forth  desirable  responses  and  to  suppress 
undesirable  ones. 

Good  and  Bad  Environment. — Only  that  environment  and 
training  are  good  which  lead  to  the  development  of  good  habits 
and  traits  and  to  the  suppression  of  bad  ones.  What  we  com- 
monly call  "good  environment"  is  frequently  the  worst  possible, 
what  is  often  called  a  bad  environment  may  be  the  best  possible. 
We  are  all  strangely  blind  with  regard  to  these  matters.  We 
know  of  many  cases  in  which  men  began  their  careers  on  a  farm, 
in  the  backwoods,  on  a  flat-boat,  amidst  hardships  and  discom- 
forts of  every  sort  and  yet  who  achieved  great  distinction.  And 
we  speak  of  such  men  as  winning  in  spite  of  disadvantages,  for- 
getting that  often  these  very  disadvantages,  hardships,  discom- 
forts, have  been  stimuli  which  have  given  them  sturdy  bodies, 
good  judgments,  good  morals,  and  have  called  forth  all  their  best 
qualities.  On  the  other  hand  under  different  circumstances  or 
with  different  men  such  conditions  may  prove  to  be  too  hard,  too 
severe,  and  the  result  be  disastrous.  But  environment  may  be 
too  good  as  well  as  too  hard.  Food  may  be  too  rich  and  too 
abundant  for  good  health,  life  may  be  too  easy  and  luxurious  for 
the  development  of  character.  Luxury,  easy  lives,  refined  sur- 
roundings have  less  of  educational  value  than  we  commonly  sup- 
pose and  they  may  be  a  positive  menace.  Any  environment  is 
bad,  however  cultured,  refined  or  pleasant  it  may  be,  which  leads 
to  the  development  of  bad  traits  of  body  or  mind.  In  general  the 


252  Heredity  and  Environment 

best  environment  is  one  which  avoids  extremes,  one  which  is 
neither  too  easy  nor  too  hard,  one  which  calls  for  sustained  effort 
and  produces  maximum  efficiency  of  body  and  of  mind. 

In  education  also  we  are  strangely  blind  as  to  proper  aims  and 
methods.  Any  education  is  bad  which  leads  to  the  formation  of 
habits  of  idleness,  carelessness  and  failure,  instead  of  habits  of 
industry,  thoroughness  and  success.  Any  religious  or  social  in- 
stitution is  bad  which  leads  to  habits  of  pious  make-believe,  insin- 
cerity, slavish  regard  for  authority  and  disregard  for  evidence, 
instead  of  habits  of  sincerity,  open-mindedness  and  independence. 

Frequently  the  training  of  the  human  being,  like  the  training  of 
a  star-fish,  consists  in  limiting  his  activities  to  particular  lines. 
Some  physical  defect  which  prevented  a  child  from  engaging  in 
the  usual  activities  of  children  has  often  turned  his  attention  to 
scholarship.  Galton  says  that  great  divines  have  usually  had  very 
poor  health.  Genius  is  frequently  associated  with  physical  de- 
fects. Great  specialization  is  associated  with  corresponding  limi- 
tations in  other  directions.  Society  needs  the.  genius  and  the 
specialist,  but  for  the  general  good  of  mankind  the  generalized 
type  is  needed  even  more  than  the  specialized. 

No  given  environment  or  training  can  be  good  for  every  in- 
dividual, nor  for  the  same  individual  at  every  stage  of  develop- 
ment. Every  individual  is  unique  and  if  the  best  results  are  to 
be  had  he  must  have  unique  environment  and  training,  which 
must  be  supplied  by  omniscient  intelligence.  Such  an  ideal  may 
not  be  practicable  but  the  impossibility  of  securing  the  absolutely 
best  conditions  of  development  need  not  prevent  society  from 
securing  better  conditions  than  those  which  now  prevail. 

Relative  Importance  of  Heredity,  Environment,  Education. — It 
is  plain  that  environment  and  education  play  a  greater  part  in  the 
development  of  man  than  in  that  of  other  animals,  whereas  hered- 
ity plays  a  similar  part ;  but  it  is  difficult  if  not  impossible  to  de- 
termine the  relative  importance  of  these  three  factors.  In  the 
field  of  intellect  and  morals  most  persons  are  inclined  to  place 


Influence  of  Environment  253 

greater  weight  upon  the  extrinsic  than  upon  the  intrinsic  factors, 
but  this  opinion  is  not  based  upon  demonstrable  evidence.  So  far 
as  organisms  below  man  are  concerned  there  is  general  agreement 
that  heredity  is  the  most  important  factor,  and  this  opinion  is  held 
also  for  man  by  those  who  have  made  a  thorough  study  of  hered- 
ity. Galton  has  made  the  best  scientific  study  of  this  subject  in 
the  case  of  identical  twins,  in  which  as  we  know  heredity  is  the 
same  in  the  two,  both  individuals  having  come  from  the  same 
oosperm  (Fig.  81).  In  bodily  and  mental  characters  such  twins 
are  remarkable  alike;  the  differences  which  exist  are  slight  and 
may  usually  be  traced  to  different  environmental  and  educational 
influences,  arid  particularly  to  different  illnesses.  Galton  sums 
up  his  study  with  these  words:  "There  is  no  escape  from  the 
conclusion  that  nature  prevails  enormously  over  nurture  when  the 
differences  of  nurture  do  not  exceed  what  is  commonly  to  be 
found  among  persons  of  the  same  rank  of  society  and  in  the  same 
country." 

A 


X 

•' 


v»  / 

^L/ 


Heredity 

FIG.  88.  DIAGRAM  TO  SHOW  THE  INFLUENCE  OF  HEREDITY,  ENVIRONMENT 
AND  TRAINING  IN  THE  DEVELOPMENT  OF  AN  INDIVIDUAL.  Various  types 
of  individuals  (represented  by  the  triangles)  may  be  produced  from  the 
same  germ  cells  (heredity)  if  the  environment  and  training  are  variable. 


254  Heredity  and  Environment 

The  part  played  by  these  different  factors  of  development  may 
be  graphically  illustrated  by  the  accompanying  diagram  (Fig.  88), 
in  which  the  base  line  represents  heredity  and  the  other  lines  rep- 
resent the  extrinsic  factors  of  environment  and  education.  For 
each  individual  heredity  is  a  constant  factor  but  environment  and 
training  are  variables.  With  a  given  heredity  the  characteristics 
of  the  developed  organism  may  vary  enormously  depending  upon 
the  extrinsic  factors.  Hereditary  possibilities  are  not  changed  by 
accidents  of  environment  but  development  is  so  changed.  After 
the  fertilization  of  the  egg  the  hereditary  potencies  of  every  organ- 
ism are  unalterably  fixed  but  the  extrinsic  factors  remain  variable 
and  may  be  controlled. 

All  of  our  social  and  ethical  institutions  such  as  government, 
education  and  religion  deal  only  with  extrinsic  factors  of  develop- 
ment and  of  life.  Nevertheless  there  is  no  evidence  that  such 
extrinsic  influences  ever  modify  heredity,  no  evidence  that  the 
effects  of  good  environment  or  good  training  ever  change  the  ger- 
minal constitution.  The  influences  of  environment  and  education 
affect  only  the  development  of  the  individual  and  not  the  consti- 
tution of  the  race,  and  consequently  such  influences  are  temporary 
in  effect  and  must  be  repeated  generation  after  generation. 

Social  versus  Germinal  Inheritance — But  though  the  effects  of 
environment  and  training  are  not  inherited,  the  environment  and 
training  and  experience  of  former  generations  are  handed  down 
to  later  generations  through  custom,  tradition,  history.  We  do 
not  inherit  through  the  germ  cells  the  effects  on  our  ancestors  of 
their  training  and  environment,  but  we  do  inherit,  in  the  property 
sense  of  that  word,  their  environment,  customs,  institutions.  In 
short  the  experiences  and  accomplishments  of  past  generations 
are  not  inherited  through  the  germ  cells  but  are  inherited  through 
society.  In  this  sense  "we  are  the  heirs  of  all  the  ages." 

Man  alone  of  all  animals  can  profit  largely  by  the  experiences 
of  others  and  especially  by  the  experiences  of  former  genera- 
tions. In  the  human  species  only  are  successive  generations  born 


Influence  of  Environment  255 

into  new  environments,  created  by  the  activities  of  preceding  gen- 
erations. To  a  certain  extent  the  young  of  higher  animals  learn 
useful  lessons  from  their  parents,  and  in  social  animals  the  en- 
vironment has  to  a  certain  extent  been  made  by  preceding  genera- 
tions, but  such  social  inheritance  is  founded  largely  on  instinct 
and  it  changes  almost,  perhaps  quite,  as  slowly  as  does  the  germ- 
plasm.  However,  when  social  inheritance  is  founded  largely  on 
intelligence  it  may  change  and  progress  very  rapidly;  intelligence 
is  a  great  time-saver  as  compared  with  instinct,  and  owinjg  to 
this  rapid  change  in  social  inheritance  every  human  generation 
is  born  into  a  new  environment,  different  from  that  of  any  pre- 
vious generation.  On  the  other  hand,  even  the  most  intelligent 
wild  animals  such  as  monkeys,  wolves,  foxes  and  elephants,  do 
not  by  their  own  activities  change  their  natural  and  social  environ- 
ments from  generation  to  generation. 

Because  of  our  social  inheritance  the  extrinsic  conditions  of  life 
continue  to  grow  more  complex  age  after  age,  while  our  inherited 
natures  remain  relatively  unchanged.  All  moralists,  all  religions, 
have  recognized  the  very  general  experience  among  men  of  a  sense 
of  imperfection  and  of  disharmony  with  social  and  ethical  stand- 
ards. Huxley  held  that  the  spirit  of  ethics  was  opposed  to  the, 
spirit  of  evolution.  Metchnikoff  finds  these  disharmonies  due  to 
the  survival  of  bestial  instincts  in  man.  Galton  finds  the  sense  of 
sin  to  be  due  to  the  fact  that  the  development  of  our  inherited  na- 
ture has  not  kept  pace  with  the  development  of  our  moral  civiliza- 
tion. Our  psychical,  social  and  moral  environment  has  come  to  us 
from  the  past  with  ever-increasing  increments,  every  age  standing 
on  the  shoulders  of  the  preceding  one.  The  aspirations,  impulses, 
responsibilities  of  modern  life  have  become  enormous  and  our 
inherited  natures  and  abilities  have  not  essentially  improved.  So- 
cial heredity  has  outrun  germinal  heredity  and  the  intellectual, 
social  and  moral  responsibilities  of  our  times  are  too  great  for 
many  men.  Civilization  is  a  strenuous  affair,  with  impulses  and 
compulsions  which  are  difficult  for  the  primitive  man  to  fulfil,  and 


256  Heredity  and  Environment 

many  of  us  are  hereditarily  primitive  men.  The  frequent  result 
is  disharmony,  poor  adjustment,  a  struggle  between  primitive  in- 
stincts and  high  ideals,  with  a  resulting  sense  of  discouragement 
and  defeat  which  often  ends  in  abnormal  states  of  mind.  The 
prevalence  of  crime,  alcoholism,  depravity  and  insanity  is  an 
ever-increasing  protest  and  menace  of  weak  men  against  high 
civilization.  We  are  approaching  the  time  when  one  or  the  other 
must  give  way,  either  the  responsibilities  of  life  must  be  reduced 
and  the  march  of  civilization  stayed,  or  a  better  race  of  men,  with 
greater  hereditary  abilities,  must  be  bred. 

Wars  and  revolutions  shake  off  the  burdens  of  social  inheritance 
but  they  destroy  the  good  along  with  the  bad  and  afford  only  lo- 
cal and  temporary  relief.  Mankind  cannot  return  permanently  to 
barbarism  or  savagery ;  civilization  must  be  and  will  be  preserved ; 
but  if  society  is  really  to  advance  from  age  to"  age  the  natures  of 
men  must  improve  as  well  as  their  environment. 


CHAPTER  V 
CONTROL  OF  HEREDITY:  EUGENICS 


CHAPTER  V 


CONTROL  OF  HEREDITY:     EUGENICS 

It  is  the  aim  of  science  to  interpret  phenomena  and  as  far  as 
possible  to  control  them.  To  what  extent  is  it  possible  to  control 
heredity  and  thus  to  improve  the  race,  as  well  as  the  individual? 

A.     DOMESTIC  ANIMALS  AND  CULTIVATED   PLANTS 

The  history  of  domesticated  animals  and  cultivated  plants 
shows  that  it  is  possible  to  control  or  rather  guide  phenomena  of 
heredity  and  evolution.  Very  many  species  of  wild  animals  have 
been  tamed  by  man  but  only  about  40  species  may  be  classed 
as  domesticated.  DeCandolle  recognized  247  species  of  cultivated 
food  plants,  193  of  which  still  exist  in  the  wild  state.*  In  a  num- 
ber of  instances  the  wild  stocks  from  which  these  domestic  forms 
came  are  known  and  it  is  possible  to  compare  them  with  their 
modified  descendants  and  thus  to  determine  the  degree  of  change 
which  has  been  brought  about  under  human  guidance.  In  other 
cases  where  the  original  wild  species  are  unknown  it  is  possible 
to  determine  the  amount  of  modification  which  has  taken  place 
within  recent  times. 

The  degree  of  change  which  has  taken  place  under  human  guid- 
ance is  very  remarkable.  In  some  cases  dozens  and  even  hun- 
dreds of  races  have  been  formed,  showing  the  most  remarkable 
differences  in  size,  structure  and  proportion  of  parts,  as  well  as 
in  functions,  instincts  and  behavior.  The  extent  to  which  hered- 

*  Bailey  says  that  more  than  20,000  kinds  of  plants  in  cultivation  are 
described  in  the  "Standard  Cyclopedia  of  Horticulture,"  although  not 
nearly  all  of  these  species  are  domesticated. 

259 


260 


Heredity  and  Environment 


ity  may  be  guided  by  man  is  forcibly  illustrated  by  our  present 
races  of  domesticated  pigeons  which  Darwin  said  would  be 
classed  by  any  naturalist  who  did  not  know  their  origin  in  not  less 
than  twenty  different  species  and  three  different  genera,  though 


FIG.  89.    RACES  OF  DOMESTIC  PIGEONS,  the  wild  rock  pigeon  being  shown 
in  the  center.     (From  Romanes,  "Darwin  and  After  Darwin.") 


Control  of  Heredity:  Eugenics 


261 


all  of  them  have  descended  from  the  wild  rock  pigeon  (Figs. 
89,  90) ;  or  by  the  numerous  races  of  dogs  varying  in  size  from 
a  toy  dog  to  a  Great  Dane  or  St.  Bernard  and  showing  almost  un- 


FIG.  90.     RACES  OF  DOMESTIC  PIGEONS,  continued.     (From  Romanes.) 


262 


Heredity  and  Environment 


SCOTCH  CRl 

FIG.  91.    RACES  OF  FOWLS.     (From  Romanes.) 


believable  differences  in  structure  and  disposition;  or  by  the 
great  variety  of  domestic  fowls,  all  of  which  have  probably  de- 
scended from  the  wild  jungle- fowl;  or  by  modern  races  of  horses, 
cattle,  sheep,  or  swine.  These  are  only  a  few  of  the  many  illus- 


Control  of  Heredity:  Eugenics 


263 


FIG.  92.     RACES  OF  FOWLS,  continued.     (From  Romanes.) 

trations  which  could  be  cited  of  the  practical  control  of  heredity 
and  evolution  for  human  purposes.  How  have  the  present  races 
of  domesticated  animals  and  cultivated  plants  been  produced? 


264 


Heredity  and  Environment 


\ 


FIG.  93.  WILD  BOAR  CONTRASTED  WITH  MODERN  DOMESTIC  PIG.  (From 
Romanes.) 

I.    THE  INFLUENCE  OF  ENVIRONMENT  IN  PRODUCING 
NEW  RACES 

There  is  a  popular  belief  that  the  improvement  of  cultivated 
races  is  due  to  good  environment,  good  food,  good  soil,  protection 
from  enemies,  etc.,  and  that  if  turned  out  to  shift  for  themselves 
such  races  revert  at  once  to  the  original  wild  stock.  This  is  not 
strictly  true  but  if  it  were  there  are  two  ways  in  which  it  is  con- 
ceivable that  new  races  might  be  produced  by  environmental 
influences : 

i.  By  the  direct  inheritance  of  somatic  or  personal  characters 
acquired  under  the  stimulus  of  the  environment.  In  spite  of  popu- 
lar opinion  in  favor  of  this  view  there  is  no  evidence  that  this 
ever  occurs.  There  is  no  doubt  that  environment  has  much  to 
do  with  individual  development,  but  it  does  not  usually  modify 
the  hereditary  constitution  of  the  race. 


Control  of  Heredity:  Eugenics 


265 


FIG.  94.     DIFFERENT  BREEDS  OF  BRITISH  CATTLE.     (From  Romanes.) 


266  Heredity  and  Environment 

2.  It  is  possible  that  environmental  changes  acting  upon  germ 
cells  at  a  sensitive  period  of  their  development  may  produce  ger- 
minal variations  or  mutations  and  thus  give  rise  to  new  races. 
But  while  this  is  possible  there  is  little  evidence  of  a  satisfactory 
nature  that  it  actually  occurs  and  there  is  no  evidence  that  such 
changes  in  hereditary  constitution  are  reversible  and  that  the 
race  reverts  to  its  former  state  when  the  old  environment  is  re- 
stored. Such  reversible  changes  undoubtedly  occur  in  somatic 
characters  but  they  are  not  inherited;  they  are  modifications  of 
development,  not  of  heredity;  they  are  personal  fluctuations,  not 
racial  mutations.  Not  infrequently  reversion  of  cultivated  races 
to  wild  stock  is  due  to  hybridization,  as  in  the  sweet  peas  de- 
scribed on  pages  102-104,  or  in  hybrids  of  some  races  of  domestic 
pigeons  (p.  82). 

II.    ARTIFICIAL  SELECTION 

Since  the  beginning  of  historic  times,  and  probably  through 
long  prehistoric  ages,  breeders  have  followed  the  method  of  se- 
lecting desirable  individuals  for  propagating  their  stock.  There 
can  be  no  doubt  that  almost  all  that  man  has  done  in  the  produc- 
tion of  domestic  animals  and  cultivated  plants  has  been  accom- 
plished by  this  process  of  selection. 

How  Has  Selection  Acted?  Darwin's  Views. — Until  very 
recently  it  was  generally  believed  that  continued  selection  of  in- 
dividuals which  showed  desirable  characters  gradually  led  to  the 
improvement  of  those  characters  and  thus  to  the  production  of 
new  races;  it  was  supposed  that  the  character  in  question  was 
"built  up"  by  continued  selection  in  one  direction,  and  that  the 
average  development  of  the  character  in  all  the  offspring  was  thus 
increased  in  successive  generations  to  an  indefinite  extent.  It 
was  this  view  as  to  the  supposed  action  of  artificial  selection 
which  formed  the  basis  of  Darwin's  theory  of  natural  selection. 

Non-effect  of  Selection  on  Pure  Lines. — On  the  other  hand  it 
has  been  known  for  a  long  time  that  the  limits  of  the  possible  im- 
provement of  any  character  by  artificial  selection  are  soon  reached 


Control  of  Heredity:  Eugenics  267 

and  that  thereafter  selection  serves  only  to  maintain  the  charac- 
ter at  its  high  level  but  not  to  advance  it.  The  probable  explana- 
tion of  this  fact  has  been  found  only  in  recent  years.  The  re- 
searches of  deVries,  Johannsen,  Jennings,  Tower,  Pearl,  Mor- 
gan and  others  have  shown  that  in  some  cases  at  least  selection 
merely  isolates  mutants  or  distinct  hereditary  lines  which  are  al- 
ready present  in  a  mixed  population  but  that  it  does  not  "build 
up"  characters  nor  produce  new  mutations;  in  short  it  does  not 
create  the  variations  on  which  it  acts. 

Johannsen  found  that  from  a  single  species  of  beans  he  was 
able,  by  keeping  the  progeny  of  each  individual  bean  separate  from 
the  others,  to  isolate  19  different  "pure  lines,"  each  differing  in 
certain  respects  from  every  other  line.  These  lines  were  not 
created  by  selection  but  were  merely  sorted  out  of  the  general 
species  where  they  existed  already.  He  further  found  that  when 
extremely  large  or  small  individuals  from  any  pure  line  were  se- 
lected and  propagated,  none  of  the  progeny  showed  that  charac- 
ter in  a  still  more  extreme  degree  but  all  merely  fluctuated  within 
the  original  extremes  of  that  line.  He  concludes  therefore  that 
selection  within  a  pure  line  is  absolutely  without  effect  in  modify- 
ing any  character  in  the  offspring  of  that  line. 

Jennings  found  that  different  races  of  Paramecium  differ  in 
size,  structure  and  rate  of  division,  and  that  these  differences  are 
"as  rigid  as  iron."  With  respect  to  average  length  of  body  he  was 
able  to  isolate  eight  lines  which  constantly  differed  more  or  less 
from  one  another.  Within  each  of  these  lines  there  was  con- 
siderable fluctuation  in  size,  but  he  was  unable  by  selecting  ex- 
tremes to  increase  these  fluctuations,  the  progeny  of  any  line  al- 
ways fluctuating  about  the  mean  of  that  line  (Fig.  22). 

Similarly  Tower  found  in  his  studies  on  the  potato  beetle  that 
he  was  unable  to  shift  the  mean  or  the  extremes  of  any  character 
by  selection  of  extreme  forms  of  an  inbred  line. 

Pearl  also  made  an  extensive  study  of  the  records  of  breed- 
ing experiments  extending  over  many  years  in  which  the  attempt 


268  Heredity  and  Environment 

was  made  to  increase  the  egg-laying  capacity  of  hens  by  selecting 
for  breeding  in  each  generation  only  those  which  had  a  high 
record  for  egg  production.  It  was  found  that  certain  "blood 
lines"  produced  a  larger  number  of  eggs  than  other  lines,  but 
by  no  amount  of  selection  was  it  possible  to  increase  the  egg  pro- 
duction within  any  line. 

On  the  other  hand  Castle  strenuously  defended  the  im- 
portance of  the  selection  of  fluctuations  in  "building  up"  a  char- 
acter. From  among  the  descendants  of  piebald  rats  and  rab- 
bits he  selected  through  many  generations  individuals  showing 
the  largest  and  others  showing  the  smallest  extent  of  color  in  the 
coat  and  he  thus  produced  one  line  which  was  nearly  all-black 
and  another  nearly  all-white.  He  maintained  that  not  only  in- 
herited characters,  but  also  their  factors  are  variable  and  that  by 
means  of  selection  of  plus  or  minus  variations  the  mode,  or  mean, 
of  a  character  may  be  shifted  in  one  direction  or  the  other.  This 
is  the  old  view  which  flourished  before  a  distinction  was  made 
between  fluctuations  and  mutations.  Breeders  have  long  been 
acquainted  with  similar  results  of  selection  from  a  mixed  popu- 
lation containing  different  hereditary  lines ;  however  Castle  was 
careful  to  employ  as  pure  a  race  of  rats  and  of  rabbits  as  he 
could  obtain,  but  it  is  not  possible  to  get  as  pure  a  race  of  these 
animals  or  of  any  organisms  in  which  cross  fertilization  occurs 
as  in  the  case  of  self-fertilizing  plants  such  as  beans.  Johannsen 
defines  a  "pure  line"  as  "all  individuals  which  are  derived  from  a 
single,  absolutely  self-fertilizing,  homozygous  individual"  and 
within  such  a  pure  line  he  maintains  that  selection  is  unable  to 
change  any  character. 

Adherents  of  the  "pure  line"  hypothesis  explain  Castle's  re- 
sults in  one  of  two  or  three  ways :  either  his  material  may  not  have 
been  genetically  pure,  or  mutations  may  have  occurred  during  the 
course  of  his  experiments  and  his  selection  served  merely  to  iso- 
late distinct  hereditary  lines  already  present;  or,  more  probably, 
extent  of  color  in  his  animals  may  depend  upon  multiple  factors, 
or  modifying  factors,  which  are  more  numerous  in  some  individu- 


Control  of  Heredity:  Eugenics  269 

als  than  in  others,  as  is  the  case  for  example  in  " blending"  inheri- 
tance (pp.  109-112),  and  selection  merely  sorted  out  some  individ- 
uals with  a  larger  or  smaller  number  of  factors,  and  consequently 
with  a  larger  or  smaller  development  of  the  character  in  question, 
but  it  did  not  in  the  least  modify  any  individual  factor.  This 
view  finds  support  in  the  work  of  McDowell,  of  Zeleny  and 
Mattoon,  and  of  Morgan  and  his  associates  on  the  effects  of  se- 
lection on  certain  characters  of  Drosophila.  In  a  recent  paper 
Castle  (1918)  himself  has  abandoned  his  former  position  and  has 
adopted  this  explanation  of  his  results,  and  thus  this  controversy 
comes  to  an  end. 

The  crux  of  this  whole  controversy  lay  in  the  question  as  to 
whether  inheritance  factors  fluctuate  or  not;  Castle  maintained 
that  they  do,  Johannsen  that  they  do  not.  If  inheritance  factors 
fluctuate  they  may  be  changed  gradually  in  one  direction  or  an- 
other by  selection;  if  they  do  not  fluctuate,  but  mutate  only, 
changing  only  rarely  and  not  in  all  directions,  selection  can  act 
only  by  sorting  out  mutations,  but  can  do  nothing  to  produce 
them.  In  general  fluctuations  are  due  to  environment  and  are 
not  inherited,  therefore  they  concern  development  rather  than 
heredity,  developed  characters  rather  than  inheritance  factors. 
There  is  much  evidence  that  inheritance  factors  are  relatively 
stable  and  that  when  they  change,  they  undergo  a  complete  change 
or  mutation  comparable  to  what  occurs  in  a  chemical  reaction.  As 
we  have  seen  it  is  possible  to  explain  almost  all  phenomena  of 
inheritance  on  this  basis  even  including  Castle's  results. 

In  another  paper  Jennings  has  shown  that  continued  selection 
in  one  direction  does,  apparently,  shift  the  mode  of  certain  char- 
acters in  a  race  of  asexually  reproducing  Difflugia,  and  Mid- 
dleton  has  found  the  same  to  be  true  of  Stylonychia.  These  re- 
sults differ  totally  from  Jennings'  earlier  work  on  Paramecium, 
which  has  been  repeated  and  confirmed  by  Ackert.  It  is  a 
hard  thing  to  believe  that  different  organisms  differ  irrecon- 
cilably in  so  fundamental  a  matter  and  it  seems  much  more  prob- 
able that  these  discrepancies  are  due  to  an  incomplete  analysis 


270 


Heredity  and  Environment 


Control  of  Heredity:  Eugenics 


271 


272  Heredity  and  Environment 

of  the  phenomena  in  question.  It  is  possible  that  the  divisions 
of  the  cell  body  in  these  protozoans  is  not  always  into  exactly 
equivalent  halves  in  which  case  variations  might  take  place  in 
the  descendants,  which  might  then  be  heaped  up  by  selection ;  or 
perhaps  there  are  multiple  or  modifying  factors  in  this  case  also, 
so  that  selection  has  acted  as  in  Castle's  rats. 

Value  of  Selection. — In  conclusion,  the  evidence  which  is  most 
clear-cut  and  abundant  indicates  that  selection  by  itself  is  unable 
to  change  inheritance  factors  or  to  cause  mutations.  Nevertheless 
selection  is  of  great  service  in  separating  good  lines  or  races  from 
poor  ones,  and  this  is  the  chief  significance  of  the  artificial  selec- 
tion practiced  by  breeders. 

The  elimination  of  certain  races  by  natural  selection  is  an  im- 
portant factor  in  evolution  though  it  has  nothing  to  do  directly  with 
the  formation  of  new  characters  or  new  races  but  serves  merely 
as  a  sieve,  as  deVries  has  expressed  it,  to  sort  the  individuals 
which  are  supplied  to  it.  Although  selection  has  no  power 
to  make  or  change  characters,  it  preserves  certain  lines  and  elim- 
inates others  and  thus  fixes  the  type  of  a  species.  Finally  the 
elimination  of  the  unfit  by  natural  selection  is  still  the  only 
mechanistic  explanation  of  fitness,  or  adaptation,  in  organisms.  . 

III.     METHODS  OF  MODERN  GENETICS 

I.  Mendelian  Association  and  Dissociation  of  Characters. — 
Breeders  have  long  known  that  it  is  possible  to  get  certain  desir- 
able characters  of  an  organism  from  one  race  and  others  from 
another  race.  But  since  the  discovery  of  the  Mendelian  princi- 
ples of  heredity  such  new  combinations  of  old  characters  have 
been  made  repeatedly,  and  with  almost  the  same  certainty  of 
results  as  when  the  chemist  makes  combinations  of  elements  or 
compounds. 

In  Fig.  95,  A  and  B,  are  shown  two  guinea-pigs,  one  having 
long  (L),  rough  and  tumbled  (T),  white  (W)  hair,  and  the  other 
having  short  (S),  smooth  (Sin),  red  (R)  hair.  When  such  ani- 
mals are  crossed  one  should  get  in  the  F2  generation  27  genotypes 


Control  of  Heredity:  Eugenics  273 

and  8  phenotypes,  one  of  each  of  the  latter  being  homozygous  and 
breeding  true,  as  is  shown  in  Fig.  32  for  trihybrid  peas. 
These  8  phenotypes  of  this  cross  are  STR,  STW,  SSmR,  SSmW, 
LTR,  LTW,  LSmR,  LSmW.  In  Fig.  95,  C  and  D,  and  Fig.  96, 
A,  B,  C,  are  shown  5  of  these  8  phenotypes  which  were  ob- 
tained by  Castle  from  this  cross.  These  figures  well  illustrate  the 
new  combinations  of  Mendelian  characters  which  may  be  obtained 
by  cross  breeding. 

Hybridization. — This  is  the  chief  method  employed  by  Bur- 
bank  in  producing  his  really  wonderful  "new  creations  in  plant 
life."  By  extensive  hybridization  he  brings  about  many  new  com- 
binations of  old  characters,  a  few  of  which  may  be  commercially 
valuable,  and  sometimes  actually  new  characters  or  mutations 
appear,  possibly  as  a  result  of  the  interaction  of  old  characters, 
or  rather  of  their  factors.  Lotsy,  for  example,  maintains  that  the 
sole  source  of  variation  is  crossing,  and  Bateson  says  that  the 
new  breeds  of  domestic  animals  made  in  recent  times  are  the 
carefully  selected  products  of  the  recombination  of  pre-existing 
breeds,  and  that  most  of  the  new  varieties  of  cultivated  plants  are 
the  outcome  of  deliberate  crossing. 

One  of  the  striking  results  of  modern  work  in  plant-breeding 
has  been  the  discovery  of  the  greatly  increased  vigor  of  certain 
hybrids  as  compared  with  either  pure-bred  parent.  In  general  it 
is  not  possible  to  tell  without  previous  experience  what  the  char- 
acter of  the  hybrid  of  two  races  or  lines  will  be;  sometimes  it 
is  more  and  sometimes  less  vigorous  than  either  parent,  but  not 
infrequently  it  is  more  vigorous.  East  and  Shull  have  shown 
that  hybrids  between  two  races  of  corn  may  be  very  much  larger 
and  more  fertile  than  either  parent.  In  some  instances  the  yield 
of  corn  per  acre  has  been  increased  from  2030  bushels  to  80-90 
bushels,  and  in  one  case  to  more  than  250  bushels  per  acre  (Figs. 
97,  .98).  Unfortunately  such  hybrid  races  of  corn  do  not  con- 
tinue to  breed  true  and  the  crossing  must  be  made  anew  in  each 
generation  if  maximum  results  are  to  be  had.  Nevertheless  this 


274 


Heredity  and  Environment 


Control  of  Heredity:  Eugenics 


275 


276  Heredity  and  Environment 

stimulation  of  hybridization  or  "heterosis,"  as  it  has-been  called, 
offers  an  extremely  important  means  of  quickly  producing  very 
vigorous  and  fruitful  individuals,  but  not  lines  or  races  which 
breed  true. 

2.  Mutations. — Mendelian  association  and  dissociation  of 
characters  produces  new  forms  of  adult  animals  and  plants  but 
not  new  hereditary  characters.  Permutations  of  Mendelian 
characters  we  may  have  almost  without  number,  of  new 
combinations  of  these  there  may  be  no  end,  but  no  new  unit  char- 
acters are  formed  by  such  temporary  combinations,  no  new  in- 
heritance factors  are  created  or  evolved.  New  combinations  of 
factors  may  be  cornpared  to  new  combinations  of  chemical  ele- 
ments ;  you  can  always  get  out  of  the  combination  what  went  into 
it,  whereas  new  factors  are  comparable  to  the  changes  which  take 
place  in  certain  atoms,  for  example  radium,  by  which  the  element 
itself  is  changed  in  an  irreversible  manner.  The  discoveries  of 
Mendel  show  us  how  to  follow  inheritance  factors  through  many 
combinations  and  through  many  generations,  but  they  do  not  show 
us  how  new  factors  arise.  These  discoveries  have  given  us  an 
invaluable  method  of  sorting  and  combining  hereditary  qualities, 
but  Mendelian  inheritance  as  such  does  not  furnish  the  materials 
for  evolution. 

In  1901  Hugo  deVries  startled  the  scientific  world  by  the  pub- 
lication of  his  great  work  on  the  "Mutation  Theory"  of  evolu- 
tion in  which  he  proved  that  the  evening  primrose,  Oenothera 
lamarckiana  occasionally  produced  "sports"  or  "mutations"  which 
differed  so  much  from  the  parent  form  that  they  deserved  to  be 
called  new  species  (Fig.  100).  He  discovered  and  studied  a 
large  number  of  these  mutations  in  Oenothera  as  well  as  in  some 
other  plants  and  concluded  that  evolution  takes  place  by  steps 
or  jumps  rather  than  by  "creeping  on  from  point  to  point"  as 
Darwin  believed. 

Several  geneticists  have  expressed  doubt  as  to  whether  there 
are  any  such  'things  as  mutations  in  the  sense  of  deVries, 


Control  of  Heredity:  Eugenics  277 

maintaining  that  his  results  may  be  explained  by  assum- 
ing that  Oenotjiera  is  a  hybrid  and  that  the  various  "mutations" 
which  he  has  described  are  due  to  segregation  and  recombi- 
nation of  old  factors  rather  than  to  the  appearance  of  new 
ones.  Indeed  Davis  has  made  up  an  Oenothera  by  hybridiza- 
tion that  is  similar  to  O.  lamarckiana.  There  are  many  things 
which  s'eem  to  indicate  that  this  species  arid  probably  other  species 
of  Oenothera  are  not  genetically  pure  and  it  is  probable  that  some 
of  deVries'  results  may  be  due  to  this  fact. 

However,  if  0.  lamarckiana  were  an  ordinary  hybrid  it  should 
show  phenomena  of  Mendelian  splitting  with  the  usual  Mendelian 
ratios.  The  fact  that  the  "mutations"  observed  by  deVries  occur 
only  very  rarely  shows  that  they  are  not  the  ordinary  emergence 
of  Mendelian  recessives.  But  the  discovery  of  lethal  factors  and 
of  "crossing  over"  in  Drosophila  has  pointed  out  a  possible  method 
of  accounting  for  many  Oenoihera  "mutations."  In  Drosophila 
it  is  known  that  when  homologous  lethal  factors  are  received  from 
both  parents  the  zygote  is  non-viable;  however,  if  the  lethal  factor 
received  from  one  parent  is  balanced  by  a  normal  one  from  the 
other  parent  the  zygote  is  viable;  in  other  words,  all  individuals 
that  are  homozygous  for  any  lethal  disappear,  while  only  those 
that  are  heterozygous  survive. 

If  recessive  factors  are  linked  with  a  lethal  they  can  not  come 
to  expression,  for  recessives  appear  only  when  mated  with  other 
recessives.  But  if  "crossing  over"  should  take  place  in  such  a  way 
as  to  break  the  linkage  between  the  lethal  and  the  recessive  fac- 
tors, the  latter  would,  when  homozygous,  come  to  expression  as 
ordinary  Mendelian  recessives. 

More  than  half  of  the  mutants  of  O.  lamarckiana  which  were 
first  described  by  deVries  are  probably  due  to  this  cause.  Shull's 
very  extensive  work  on  this  species  shows  that  it  is  a  persistent 
heterozygote  (hybrid)  in  which  certain  recessive  characters  do 
not  appear  as  in  ordinary  Mendelian  segregation,  because  the  fac- 
tors for  these  characters  are  linked  with  a  lethal  factor  and  it  is 


Heredity  and  Environment 


H  e 

in     O 

"8* 


Control  of  Heredity:  Eugenics,  279 

only  when  this  linkage  is  broken  that  they  have  a  chance  to 
develop. 

However,  it  is  certain  that  mutations  do  take  place  in  species 
where  there  is  no  evidence  of  genetic  impurity,  as  for  example  in 
Drosophila  melanoga-ster,  and  it  is  an  extraordinary  circumstance 
that  many  of  the  mutations  of  Oenothera  upon  which  deVries 
founded  his  great  theory  are  probably  not  mutations  at  all  but  are, 
as  Muller  (1917)  has  said,  "merely  the  emergence  into  a  state  of 
homozygosis,  through  crossing  over,  of  recessive  factors  constantly 
present  in  the  heterozygous  stock."  DeVries  himself  had  pre- 
viously suggested  this  explanation  for  his  double  reciprocal  crosses 
and  as  Muller  says,  "it  probably  lies  at  the  root  of  nearly  all  the 
unusual  genetic  phenomena  of  this  genus."  That  there  are  lethal 
factors  also  in  Oenothera  which  produce  their  effect  upon  the 
gametes  rather  than  upon  the  zygotes  is  indicated  by  the  partial 
or  complete  failure  to  form  fertile  pollen  in  certain  forms  of 
this  genus. 

It  is  probable  that  many  natural  or  Linnean  species,  other  than 
O.  lamarckiana,  are  not  pure  and  homozygous;  within  every  such 
species  there  are  usually  found  many  "elementary  species"  and 
by  intercrossing  of  these  a  mixture  of  many  lines  or  strains  re- 
sults from  which  new  forms  may  occasionally  arise  by  segre- 
gation. Lotsy  maintains  that  all  mutations  arise  in  this  way. 
But  such  an  explanation  does  not  account  for  the  existence  of 
the  original  "elementary  species"  and  if  they  be  referred  to  still 
earlier  crossings  it  is  evident  that  we  only  put  off  the  explanation 
to  a  more  remote  period.  After  all,  the  fundamental  problem 
here  concerns  the  origin,  rather  than  the  segregation,  of  domi- 
nants, recessives,  lethals,  etc.  The  exact  and  exhaustive  work  of 
Morgan  and  his  associates  on  Drosophila  has  proved  that  the  mu- 
tations in  this  species  are  not  due  to  Mendelian  segregation  and  it 
plainly  indicates  that  they  are  caused  by  sudden  transformations 
in  the  Mendelian  factors  themselves,  comparable  to  changes  in 
chemical  composition. 


280  Heredity  and  Environment 

3.  Causes  of  Mutations.  The  causes  of  new  hereditary  char- 
acters, or  rather  of  mutations  in  genes,  are  obscure.  Practically 
all  of  the  earlier  workers  and  writers  on  evolution  found  the 
principal  causes  of  transmutation  in  the  action  of  extrinsic  or 
environmental  forces  on  the  organism.  As  the  result  of  years  of 
labor  on  this  subject  Darwin  concluded  that  "variability  of  every 
sort  is  due  to  changed  conditions  of  life" ;  but  in  the  light  of  mod- 
ern genetics  such  a  statement  is  too  sweeping. 

It  is  well  known  that  environmental  changes  produce  many 
kinds  of  modifications  in  organisms,  and  in  general  these  modi- 
fications are  the  more  profound  the  earlier  they  occur  in  ontog- 
eny; it  is  known  that  slight  alterations  of  the  germ  cells  may 
produce  great  modifications  of  adult  structure,  and  yet  one  of  the 
most  striking  results  of  recent  work  is  to  show  the  small  effect 
of  environmental  changes  on  hereditary  characters.  Marked  in- 
dividual modifications  may  be  produced  which  do  not  become 
racial.  Usually  not  one  of  thousands  of  variations  which  occur 
has  any  evolutionary  value.  These  fluctuations  come  with  chang- 
ing environment  and  with  changing  environment  they  disappear. 
Very  rarely  a  sudden  variation  or  mutation  of  the  germplasm 
arises  which  forms  the  basis  of  a  new  race  (Figs.  99,  100,  101). 
In  most  cases  such  mutations  manifest  themselves  in  the  failure 
of  some  old  character  to  develop  rather  than  in  the  addition  of 
a  new  one,  but  at  least  they  represent  modifications  of  hereditary 
constitution  and  as  such  they  furnish  material  for  evolution. 
Whence  and  how  they  appear  we  do  not  know,  for  like  the  king- 
dom of  heaven  they  come  without  observation.  Their  infrequency 
amidst  the  multitude  of  fluctuations  indicates  the  wonderful  sta- 
bility of  racial  types  and  teaches  respect  for  Weismann's  doctrine 
of  a  germplasm  relatively  stable,  independent  and  continuous. 

This  distinction  between  somatic  and  germinal  variations,  be- 
tween those  which  concern  only  the  individual  and  those  which 
are  inherited  and  furnish  material  for  evolution,  marks  the  great- 
est advance  in  the  study  of  evolution  since  the  work  of  Darwin. 
And  just  as  these  germinal  variations  are  the  only  ones  of  impor- 


Control  of  Heredity:  Eugenics 


281 


FIG.  100.  COMMON  COLORADO  POTATO  BEETLE,  Leptinotarsa  and  some  of 
its  Mutants,  a,  normal  undecemlineata,  b,  the  mutant  augusto-vittata,  c, 
the  mutant  mclanothorax ,  d,  normal  decemlineata,  e,  the  mutant  tortuosa,  f, 
the  mutant  defectopunctata.  (From  Plate  after  Tower.) 

tance  in  the  process  of  evolution  so  the  question  of  their  origin 
is  the  greatest  evolutionary  problem  of  the  present  day.  How 
are  such  germinal  variations  produced? 

There  is  some  evidence  that  environmental  changes  of  the 
right  sort  acting  upon  germplasm  at  the  right  stage  may  lead  to 
permanent  modification  of  the  hereditary  organization.  Extrin- 
sic influences  acting  upon  germ  cells  at  the  time  of  their  matura- 
tion divisions  may  lead  to  new  distributions  of  chromosomes  or 
even  to  changes  in  the  composition  of  individual  chromosomes, 
thus  producing  new  hereditary  types.  Certain  mutants  of  Oeno- 


282  Heredity  and  Environment 

thera  (Fig.  99)  seem  to  be  of  this  sort  as  Gates,  Miss  Lutz  and 
Stomps  especially  have  emphasized;  for  example  O.  lamarckiana 
has  14  chromosomes,  O.  lata,  O.  albida  and  O.  scintillans  15  each, 
O.  semi-gigas  21,  O.  gigas  28,  and  these  variations  in  the  number 
of  'chromosomes  are  probably  due  to  abnormalities  in  their  divi- 
sions, such  as  non-disjunction,  irregular  distribution,  or  chromo- 
some division  without  cell  division.  It  is  significant  that  the 
mutants  lata  and  semi-gigas  have  occurred  several  times,  whereas 
gigas  appeared  but  once;  this  may  be  explained  by  the  fact  that 
the  chances  of  the  doubling  of  chromosomes  in  both  germ  cells 
(gigas)  are  very  few  compared  with  the  chances  of  their  doub- 
ling in  one  germ  cell  (semi-gigas)  or  of  their  increase  by  one  in 
a  single  germ  cell  (lata). 

But  it  is  probable  that  mutations  are  not  usually  associated  with 
changes  in  the  number  of  chromosomes.  Where  the  number  of 
chromosomes  remains  constant  the  change  may  take  place  in  the 
composition  of  single  chromosomes,  as  in  "cross-overs"  or  in  the 
number  or  composition  of  the  chromomeres  or  units  of  the  next 
lower  order.  But  such  changes  as  these  concern  only  the  emer- 
gence into  visibility  of  more  fundamental  changes,  for  whatever 
the  cellular  changes  may  be  which  accompany  mutations,  it  is 
certain  that  the  fundamental  changes  take  place  in  the  inheritance 
factors  themselves. 

In  addition  to  gene  mutations,  such  as  the  transformation  of 
the  gene  for  the  red  eye  of  Drosophila  into  one  for  white  eye, 
some  genes  appear  to  have  become  inactive  or  to  have  dropped 
out  altogether,  as  in  the  white  sweet  peas  shown  in  Fig.  33,  in 
the  wingless  or  eyeless  mutants  of  Drosophila  (Figs.  101-103), 
and  in  many  other  cultivated  races  of  plants  and  animals,  thus 
producing  regressive  mutations.  Indeed  most  of  our  domestic 
animals  and  cultivated  plants  appear  as  if  they  had  arisen  by  the 
omission  of  something  which  their  wild  ancestors  had.  It  was 
this  appearance  of  omission  which  led  to  the  "presence  and  ab- 
sence hypothesis"  (p.  97)  which  has  now  been  abondoned.  Phe- 


Control  of  Heredity:  Eugenics  283 

nomena  of  "multiple  allelomorphism"  (p.  98)  prove  that  recessive 
characters  are  not  the  result  merely  of,  the  absence  of  dominant 
ones.  On  the  other  hand  many  things  suggest  that  recessive 
genes  may  originate  from  dominant  ones  by  a  process  of  de- 
gradation. In  many  of  the  mutations  studied  by  deVries,  Bate- 
son,  Morgan  and  others  some  factor  seems  to  have  undergone 
degradation  and  Bateson  suggests  that  at  present  all  new  forms 
arise  only  by  the  loss  of  factors  or  by  the  fractionation  of  factors 
and  that  new  factors  are  not  added  from  without.  This  leads  him 
to  inquire  "whether  the  course  of  evolution  can  at  all  reasonably 
be  represented  as  an  unpacking  of  an  original  complex  which 
contained  within  itself  the  whole  range  of  diversity  which  living 
things  present."  This  is  as  extreme  preformation  in  the  field  of 
inheritance  factors  as  was  the  old  theory  of  "emboitement"  in 
the  field  of  developed  characters ;  it  is  devolution  rather  than  evo- 
lution since  it  assumes  that  the  earliest  organisms  were  geneti- 
cally the  most  complex. 

But  if,  as  is  often  said,  evolution  from  amoeba  to  man  neces- 
sarily involves  the  addition  of  many  new  inheritance  factors  it 
does  not  involve  additions  from  without  as  Bateson  implies.  New 
hereditary  factors  are  to  be  thought  of  as  we  think  of  new  chemical 
compounds,  which  are  formed  by  new  combinations  of  the  same 
old  elements,  or  as  we  think  of  new  elements,  such  as  helium  and 
radium  emanation,  which  are  formed  by  dissociation  of  radium. 
As  compared  with  chemical  elements  the  factors  of  heredity  are 
probably  very  complex  things  and  *lr  ^°w  factors  which  appear  in 
the  course  of  evolution  probably  arise  as  new  combinations  of 
factors  or  parts  of  factors  previously  present.  Nowhere  in  the 
entire  process  of  organic  evolution  is  there  any  evidence  that 
new  factors  are  "extrinsic  additions"  or  are  created  de  novo. 
The  whole  process  is  one  of  evolution,  that  is  of  new  combina- 
tions of  existing  units,  having  new  qualities  which  are  the  re- 
sults of  these  new  combinations. 

If  these  changes  in  the  germplasm  may  be  induced  by  extrinsic 


Heredity  and  Environment 


FIG.  101.  TYPICAL  AND  MUTANT  FORMS  OF  Drosophila  melanogaster, 
A,  Typical  Female.  B,  Typical  Male.  C  to  H,  Mutants  resulting  from 
mutations  of  genes  in  Chromosome  Pair  I  (XX  or  XY)  at  "Loci"  indi- 
cated. C,  "Rudimentary"  Wing  (54-5).  D,  "Miniature"  Wing  (36.0). 
E,  "Cut"  Wing  (20.0).  F,  "Forked"  Bristles  (56.5)  and  "Bar"  Eye 
(57.0).  G,  "White"  Eyes  (1.5).  H,  "Bar"  Eye  (57.0)  seen  from  left  side. 
("Figures  loaned  by  Professor  Morgan.) 


Control  of  Heredity:  Eugenics 


285 


FIG.  102.  MUTANTS  OF  Drosophila  melanogaster  resulting  from  muta- 
tions of  genes  in  Chromosome  Pair  II  at  Loci  indicated.  A,  "Vestigial" 
Wing  (65.0).  B,  "Strap"  Wing  (65.0,  allelomorph  of  "Vestigial").  C, 
"Flipper"  Wing  (±28.0).  D,  "Squat"  Body  (±35.0).  E,  "Truncate" 
Wing  (9.0).  F,  "Arc"  Wing  (97.5).  (Figures  loaned  by  Professor 
Morgan.) 


286 


Heredity  and  Environment 


FIG.  103.  MUTANTS  OF  Drosophila  melanogaster.  A-C,  resulting  from 
mutations  of  genes  in  Chromosome  Pair  III;  E-G,  due  to  mutant  genes 
in  Chrome-some  Pair  IV  at  the  Loci  indicated.  A,  "Bithorax"  (III,  54.5). 
B,  "Ski"  Wings  (III,  43.5).  C.  "Delta"  veins  in  Wings  (III,  63.5).  D, 
"Roof"  Wings  (II,  ±77.0).  E,  "Bent"  Wings  (IV,  o.o).  F,  "Shaven", 
few  hairs  (IV,  ±0.5).  G,  "Eyeless"  (IV,  i.o)  left,  right,  and  dorsal 
views.  (Figures  loaned  by  Professor  Morgan.) 


Control  of  Heredity:  Eugenics  287 

conditions,  then  a  real  experimental  evolution  will  be  possible; 
if  they  cannot  be  so  induced  but  are  like  the  changes  taking  place 
in  the  radium  atom  we  can  only  look  on  while  the  evolutionary 
processes  proceed,  selecting  here  and  there  a  product  which 
nature  gives  us  but  being  unable  to  initiate  or  control  these  pro- 
cesses. 

'  B.     CONTROL  OF  HUMAN  HEREDITY :     EUGENICS 

^ 

I.     PAST  EVOLUTION  OF  MAN 

There  is  every  evidence  that  man  also,  no  less  than  domesti- 
cated animals,  has  evolved  from  a  natural  or  wild  state.  The 
most  primitive  types  of  men  are  known  only  from  a  few  fossil 
remains,  which  indicate  that  these  primitive  men  belonged  to  dif- 
ferent species,  and  some  of  them  even  to  different  genera,  from 
Homo  sapiens  (Fig.  104).  Later  stages  in  the  evolution  of  man 
are  known  from  many  remains,  implements  and  handiwork,  as 
well  as  from  certain  primitive  races  or  tribes  which  have  per- 
sisted to  the  present  time.  The  grades  of  culture  represented  by 
these  extinct  or  persistent  tribes  and  by  modern  men  are  usually 
classified  as  savagery,  barbarism  and  civilization.  There  must 
have  been  much  greater  evolution  of  human  types  during  pre- 
historic times  than  since  the  beginnings  of  civilization.  The 
physical,  mental  and  moral  changes  which  took  place  in  men  from 
the  earliest  stages  of  savagery  down  to  the  beginnings  of  civili- 
zation were  very  great,  but  they  were  nevertheless  slight  com- 
pared with  the  tremendous  changes  which  must  have  occurred  in 
those  long  ages  before  the  ancestors  of  man  actually  became  men. 
Within  the  historic  period  the  evolutionary  changes  in  man  have 
been  very  small.  Minor  changes  have  occurred  and  are  still  going 
on,  as  Osborn  has  shown  in  his  "Cartwright  Lectures  on  Contem- 
porary Evolution  in  Man,"  but  the  species  has  remained  relatively 
stable  during  the  historic  epoch  as  compared  with  the  much  longer 
prehistoric  period. 


288 


Heredity  and  Environment 


Control  of  Heredity:  Eugenics  289 

The  past  history  of  man  has  been  a  long  one,  no  one  can  say 
how  long,  but  probably  not  less  than  half  a  million  years  have 
passed  since  the  Hominidae  appeared,  and  not  less  than  fifty 
thousand  years  since  the  present  species  arose.  There  is  every 
reason  to  believe  that  the  future  history  of  man  will  be  even 
longer.  Barring  great  secular  changes,  catastrophes  or  cataclysms, 
which  cannot  be  foreseen  nor  provided  against,  man  controls  his 
own  destiny  on  this  planet. 

It  is  a  curious  fact  that  in  prescientific  times  the  instability  of 
nature  especially  appealed  to  men.  How  often  in  the  past  have 
men  looked  forward  to  a  "speedy  end  of  the  world"!  It  may 
well  have"  seemed  to  our  ancestors  a  useless  thing  to  take  any 
thought  for  the  morrow  if  very  soon  the  heavens  are  to  be  rolled 
up  as  a  parchment  and  the  elements  dissolved  in  fervent  heat;  it 
would  be  folly  to  plan  for  future  ages  if  the  time  is  at  hand  when 
the  angel  shall  stand  with  one  foot  on  the  sea  and  the  other  on 
land  and  declare  that  time  shall  be  no  more.  But  science  has 
taught  us  something  of  the  wonderful  stability  of  nature,  some- 
thing of  the  immensity  of  past  time  and  of  future  ages,  some- 
thing of  the  eternity  of  natural  processes.  Compared  with  this 
infinite  stability  and  eternity  of  nature  what  are  our  little  sys- 
tems and  customs!  Our  years  and  centuries  fall  like  grains  of 
sand  into  this  abyss  of  time.  Our  individual  lives  are  like  drops 
of  water  in  this  great  ocean  of  life.  What  intellectual  develop- 
ments, what  social  institutions,  what  control  of  natural  processes 
may  come  in  the  long  ages  of  futurity  it  has  not  entered  into  the 
heart  of  man  to  conceive.  And  yet  so  far  as  we  may  judge  by 
the  small  portion  of  the  record  of  the  past  which  we  can  read  there 
has  been  no  necessary  progress.  There  has  been  "eternal  process 
moving  on,"  but  not  eternal  progress.  Stagnation,  degeneration, 
elimination,  as  well  as  progression,  have  occurred  all  along  the 
path  of  evolution.  And  yet  on  the  whole  evolution  has  been  pro- 
gressive and  there  is  no  reason  to  suppose  that  the  elimination  of 


290  Heredity  and  Environment 

the  unfit  and  the  preservation  of  the  fit  will  cease  to  be  the  law 
of  future  evolution,  as  it  has  been  of  the  past. 

Existing  Human  Types. — There  are  three  principal  types  of  the 
human  species — white,  yellow,  and  black — and  many  subtypes 
and  races.  These  types  and  races  differ  in  many  regards  in  physi- 
cal, mental  and  social  characteristics,  and  their  comparative  value 
has  frequently  been  discussed.  It  is  difficult  to  take  an  impartial 
view  of  such  a  matter,  though  I  suppose  there  would  be  little 
question  on  the  part  of  any  well  informed  person  that  the  white 
and  yellow  types  have  contributed  most  to  what  we  call  civili- 
zation. Nevertheless  every  race  probably  has  good  qualities  which 
could  be  made  of  service  to  society.  The  various  races  of  cattle, 
horses,  sheep,  etc.,  are  all  useful  to  man,  but  in  different  ways 
and  degrees,  and  the  same  is  true  of  various  races  of  men  with 
respect  to  civilization.  In  general  the  dominant  races  are  the 
most  capable  'intellectually  and  socially,  while  those  which  have 
been  left  behind  or  have  been  eliminated  have  been  the  less  capa- 
ble ones.  And  yet  some  very  good  races,  possibly  with  capaci- 
ties for  high  social  and  intellectual  development,  have  been  com- 
pletely exterminated,  as  for  instance  the  Lucayan  Indians  of  the 
West  Indies,  and  the  aborigines  of  Tasmania. 

Race  Extermination. — Few  animals  have  suffered  more  whole- 
sale destruction  than  have  the  more  primitive  races  of  men  in  dif- 
ferent parts  of  the  earth.  Several  species  of  man  have  become 
entirely  extinct,  leaving  only,  as  is  generally  believed,  a  single 
existing  species,  Homo  sapiens.  Race  extermination  has  been 
witnessed  in  relatively  recent  times  and  on  a  large  scale  in  the 
West  Indies,  North  and  South  America,  Africa,  Australia,  New 
Zealand  and  the  Islands  of  the  Pacific.  But  in  the  disappearance 
of  native  races  extermination  is  usually  supplemented  by  amalga- 
mation. After  the  most  warlike  members  of  a  race  have  been 
destroyed  the  more  peaceful  remnants  are  generally  incorporated 
in  the  conquering  race.  Thus  the  Maoris  of  New  Zealand,  the 
finest  native  race  with  which  the  English  have  come  in  contact  in 


Control  of  Heredity:  Eugenics  291 

their  colonies,  were  estimated  to  number  more  than  a  quarter  of 
a  million  at  the  end  of  the  eighteenth  century.  Owing  to  im- 
ported diseases  and  to  destructive  wars  among  the  tribes  and 
with  the  English  there  are  not  fifty  thousand  of  them  today,  and 
these  are  gradually  being  absorbed  into  the  white  race. 

Undoubtedly  there  has  been  a  great  growth  of  altruism  in  the 
modern  world;  there  is  a  relatively  new  feeling  among  men  that 
nothing  so  becomes  a  strong  nation  as  the  exercise  of  justice 
toward  weaker  ones,  and  many  idealists  maintain  that  every  race 
and  every  people  has  the  right  to  live  its  life  in  its  own  way.  But 
however  philanthropic  they  may  be  in  theory,  the  practice  of  all 
nations  demonstrates  that  weaker  and  inferior  peoples  are  not 
permitted  to  stand  in  the  way  of  dominant  ones.  When  such 
peoples  occupy  territory  which  is  desired  by  more  powerful 
neighbors,  they  are  either  exterminated,  expelled,  exploited  or 
amalgamated  with  the  conquering  race.  In  practice  their  rights 
are  usually  of  small  concern  as  compared  with  the  desires  of  the 
invaders,  and  the  inaccessible  or  undesirable  parts  of  the  earth, 
the  deserts  and  mountains  and  regions  of  polar  ice,  become  the 
refuge  of  the  less  capable  races,  just  as  "the  conies,  who  are  but  a 
feeble  folk,  make  their  houses  in  the  rocks."  This  is  an  illustra- 
tion of  the  great  law  of  evolution,  the  survival  of  the  strong  and 
capable  and  intelligent,  and  even  though  ideal  justice  be  meted 
out  to  weaker  peoples,  dominant  races  will  still  dominate  and 
possess  the  earth.  The  only  recourse  which  the  inferior  peoples 
have,  and  it  is  a  terrible  revenge,  is  to  amalgamate  with  the  super- 
ior race  and  thus  lower  its  hereditary  qualities. 

From  the  way  in  which  primitive  races  have  gone  down  before 
more  cultured  ones  there  is  reason  to  believe  that  in  general  the 
principle  of  the  elimination  of  the  unfit  and  the  survival  of  the 
fit  has  characterized  human  evolution  no  less  than  that  of  other 
organisms.  Undoubtedly  intelligence  has  played  a  great  part  in 
the  evolution  of  man,  as  is  at  once  apparent  when  we  consider  the 
infinitely  varied  experiments  by  which  he  has  worked  his  way 


292  Heredity  and  Environment 

from  savagery  to  civilization.  And  yet  he  has  not  consciously  set 
before  himself  an  evolutionary  goal  to  be  attained  by  intelligent 
attention  to  principles  of  good  breeding. 

II.     CAN  HUMAN  EVOLUTION  BE  CONTROLLED? 

Almost  all  that  man,  now  is  he  has  come  to  be  without  conscious 
human  guidance.  If  evolution  has  progressed  from  the  amoeba  to 
man  without  human  interference,  if  the  great  progress  from  ape- 
like men  to  the  most  highly  civilized  races  has  taken  place  without 
conscious  human  control,  the  question  may  well  be  asked,  Is  it 
possible  to  improve  on  the  natural  method  of  evolution?  It  may 
not  be  possible  to  improve  on  the  method  of  evolution  and  yet 
by  intelligent  action  it  may  be  possible  to  facilitate  that  method. 
Man  cannot  change  a  single  law  of  nature  but  he  can  put  himself 
into  such  relations  to  natural  laws  that  he  can  profit  by  them. 

i.  Selective  Breeding  the  only  Method  of  Improving  the  Race. 
— It  is  surely  not  possible  to  'improve  on  nature's  principle  of  elim- 
inating the  worst  lines  from  reproduction.  This*  has  been  the  chief 
factor  in  the  establishment  of  races  of  domesticated  animals  and 
cultivated  plants,  though  as  we  have  seen  it  has  probably  had 
nothing  to  do  with  the  origin  of  mutations.  The  history  of  such 
races  shows  that  evolution  may  be  guided  to  human  advantage 
by  intelligent  elimination  and  selection,  and  probably  any  heredi- 
tary improvement  of  the  human  race  must  be  accomplished  by 
this  means,  though  of  course  such  elimination  and  selection  can 
apply  only  to  the  function  of  reproduction.  The  method  of  evo- 
lution by  the  elimination  of  persons,  the  destruction  of  the  weak 
and  cowardly  and  antisocial,  which  was  the  method  practiced  in 
ancient  Sparta,  is  repugnant  to  the  moral  sense  of  enlightened 
men  and  cannot  be  allowed  to  act  as  in  the  past;  but  the  worst 
types  of  mankind  may  be  prevented  from  propagating,  and  the 
best  types  may  be  encouraged  to  increase  and  multiply.  This  is 
apparently  the  only  way  in  which  we  may  hope  to  improve  per- 
manently the  human  breed. 


Control  of  Heredity:  Eugenics 

2.  No  Improvement  in  Human  Heredity  within  Historic 
Times. — The  improvement  of  environment  and  of  opportunity  for 
individual  development  enables  men  at  the  present  day  to  get 
more  out  of  their  heredity  than  was  possible  in  the  past.  Advance 
of  civilization  has  meant  only  improvement  of  environment.  But 
neither  environment  nor  training  has  changed  the  hereditary  ca- 
pacities of  man.  There  has  been  no  perceptible  improvement  in 
human  heredity  within  historic  times,  nothing  comparable  with 
the  changes  which  have  occurred  in  domesticated  animals.  In- 
deed no  modern  race  of  men  is  the  equal  of  certain  ancient  ones. 
Galton  has  pointed  out  the  ¥act  that  in  the  little  country  of  At- 
tica in  the  century  between  530  and  430  B.C.  there  were  pro- 
duced fourteen  illustrious  men,  one  for  every  4,300  of  the  free 
born,  adult  male  population.  In  the  two  centuries  from  500300 
B.  C.  this  small,  barren  country  with  an  area  and  total  popula- 
tion about  equal  to  that  of  the  present  State  of  Rhode  Island  but 
with  less  than  one-'fifth  as  many  free  persons  produced  at  least 
twenty-five  illustrious  men.  Among  statesmen  and  commanders 
there  were  Miltiades,  Themistocles,  Aristides,  Cimon,  Pericles, 
Phocion;  among  poets  ^Eschylus,  Euripides,  Sophocles,  Aristo- 
phanes ;  among  philosophers  and  men  of  science  Socrates,  Plato, 
Aristotle,  Demetrius,  Theophrastus ;  among  architects  and  artists 
Ictinus,  Phidias,  Praxiteles,  Polygnotus ;  among  historians  Thucy- 
dides  and  Xenophon;  among  orators  ^Eschines,  Demosthenes, 
Isocrates,  Lysias.  In  this  small  country  in  the  space  of  two  cen- 
turies there  appeared  such  a  galaxy  of  illustrious  men  as  has 
never  been  found  on  the  whole  earth  in  any  two  centuries  since 
that  time. 

These  illustrious  men  came  from  a  remarkable  race  composed 
of  individuals  drawn  together  from  all  the  shores  of  the  Mediter- 
ranean by  a  process  of  unconscious  but  severe  selection.  Athens 
was  the  intellectual  and  social  capital  of  the  world  and  to  it  the 
most  ambitious  and  most  capable  men  were  irresistibly  drawn. 
It  was  good  immigration  as  well  as  good  native  stock  that  made 


294  Heredity  and  Environment 

Athens  famous.  Galton  concludes  that  the  average  ability  of  the 
Athenian  race  of  that  period  was,  on  the  lowest  estimate,  as  much 
greater  than  that  of  the  English  race  of  the  present  day  as  the 
latter  is  above  that  of  the  African  negro. 

But  this  marvellously  gifted  race  declined,  as  all  such  races 
have  in  time  declined : 

Social  morality  grew  exceedingly  lax,  marriage  became  unfash- 
ionable and  was  avoided,  many  of  the  more  ambitious  and  ac- 
complished women  were  avowed  courtesans  and  consequently  in- 
fertile, and  the  mothers  of  the  incoming  population  were  of  a 
heterogeneous  class.  ...  It  can  be  therefore  no  surprise  to  us, 
though  it  has  been  a  severe  misfortune  to  humanity,  that  the 
high  Athenian  breed  decayed  and  disappeared,  for  if  it  had  main- 
tained its  excellence  and  had  multiplied  and  spread  over  large 
countries,  displacing  inferior  populations  (which  it  well  might 
have  done,  for  it  was  naturally  very  prolific),  it  would  assuredly 
have  accomplished  results  advantageous  to  human  civilization  to 
a  degree  that  transcends  our  powers  of  imagination.  (Galton, 
''Hereditary  Genius,"  page  331.) 

Bateson  suggests  that  the  high  intellectual  qualities  of  the  an- 
cient Athenian  race  were  due  to  the  inbreeding  of  homogeneous 
and  very  superior  phratries  and  gentes,  but  when  foreign  mar- 
riages were  sanctioned,  and  aliens  and  manumitted  slaves  were 
admitted  to  citizenship  by  the  "reforms"  of  Cleisthenes  (507 
B.  C.)  the  population  gradually  became  mongrelized  and  its  in- 
tellectual superiority  declined. 

3.  Why  the  Race  Has  Not  Improved. — If  mankind  has  made 
no  progress  in  hereditary  characteristics  since  the  time  of  the 
Greeks  the  cause  is  not  far  to  seek.  There  have  been  gifted  races 
and  families,  doubtless  many  notable  human  mutations  have  oc- 
curred, but  most  of  these  have  been  diluted,  squandered,  lost. 
There  has  been  persistent  violation  of  all  principles  of  good 
breeding  among  men.  For  example,  there  has  been  for  ages  a 
futile  reliance  upon  good  environment  to  improve  heredity.  Men 
do  not  so  improve  the  races  of  animals  and  plants,  and  thousands 


Control  of  Heredity:  Eugenics  295 

of  years  of  human  history  show  that  this  method  is  of  no  avail 
in  improving  the  human  breed. 

But  the  case  is  far  worse  than  this ;  such  efforts  though  futile 
are  at  least  well  intentioned,  but  on  the  part  of  most  men  and 
governments  there  has  been  complete  disregard  of  the  entire 
question  of  the  improvement  of  the  human  stock.  Natural  se- 
lection which  has  through  countless  ages  eliminated  the  worst 
and  conserved  the  best-fitted  and  thus  has  led  on  the  whole  to 
the  survival  of  the  fit  is  so  far  as  possible  nullified  by  civilized 
man;  the  worst  are  preserved  along  with  the  best  and  all  are 
given  the  same  chance  of  reproduction.  The  mistake  has  been  not 
in  nullifying  natural  selection  by  preserving  the  weak  and  incom- 
petent, for  civilized  men  could  not  well  do  otherwise,  but  in  fail- 
ing to  substitute  intelligent  artificial  selection  for  natural  selection 
in  the  propagation  of  the  race.  Instead  of  this  there  has  been 
perpetuation  of  the  worst  lines  through  sentimental  regard  for 
personal  rights,  even  when  opposed  to  the  welfare  of  society ;  and 
both  church  and  state  have  cheerfully  given  consent  and  blessing 
to  the  marriage  and  propagation  of  idiots  and  of  diseased,  defec- 
tive, insane  and  vicious  persons.  Finally  there  has  been  extinc- 
tion of  the  world's  most  gifted  lines  by  enforced  celibacy  in  many 
religious  orders  and  societies  of  scholars;  by  almost  continuous 
wars  which  have  taken  the  very  best  blood  that  was  left  outside  of 
the  monastic  orders;  by  luxury  and  voluntary  sterility;  by  vice, 
disease  and  consequent  infertility. 

Is  it  any  wonder  that  the  inheritance  of  the  human  race  has 
not  improved  within  historic  times?  Is  it  not  rather  an  evidence 
of  the  broadcast  distribution  of  good  and  wholesome  qualities  in 
the  race  that  in  spite  of  such  serious  violations  of  the  principles 
of  good  breeding  mankind  remains  as  good  as  we  find  it  today? 

III.    EUGENICS 

If  a  superior  power  should  deal  with  man  as  man  deals  with 
domestic  animals  no  doubt  great  improvement  could  be  effected 


296  Heredity  and  Environment 

in  the  human  breed.  Society  is  in  some  respects  such  a  power 
and  can  do  what  the  individual,  because  of  self-interest,  short  life 
or  lack  of  ability,  cannot  accomplish.  In  matters  of  public  health 
and  comfort,  security  of  life  and  property  society  is  superior  in 
power  to  the  individual;  in  matters  of  the  perpetuation  of  the  race 
the  individual  is  still  supreme.  In  animal  societies  the  race,  the 
breed,  is  to  the  swift  and  strong  and  fit,  and  the  same  was  prob- 
ably true  of  primitive  men.  But  it  is  impossible  to  return  to  the 
conditions  of  primitive  society  in  this  respect,  and  the  social 
body  itself  must  in  some  way  control  the  breeding  of  men. 

There  are  millions  of  men  in  civilized  countries  whose  mental 
equipment  places  them  on  a  plane  with  barbarians  or  savages, 
and  they  have  on  the  average  more  offspring  than  their  civilized 
contemporaries.  There  are  millions  of  others  who  are  so  ser- 
iously defective  in  body  or  mind,  owing  to  hereditary  causes,  that 
they  can  never  take  care  of  themselves  and  must  always  be  a 
charge  upon  the  state,  and  yet  in  many  civilized  countries  they 
are  permitted  to  perpetuate  their  kind  and  produce  a  never-ending 
supply  of  mental  and  moral  defectives,  whose  maintenance  must 
seriously  interfere  with  the  proper  education  and  development  of 
the  normal  population  and  whose  unrestrained  existence  con- 
stantly threatens  to  pollute  purer  streams  of  heredity.  The  prac- 
tice of  society  regarding  marriage  and  reproduction  up  to  the 
present  has  been  to 'allow  all  sorts,  good,  bad  and  indifferent,  to 
propagate  with  the  belief  that  good  environment  and  training  will 
make  up  for  deficiencies  of  birth.  But  recently  the  conviction 
has  been  growing  that  good  environment  is  far  less  important 
than  good  heredity  and  that  in  some  way  society  must  influence 
the  race  of  men  at  its  source.  This  is  the  doctrine  of  eugenics, 
which  Galton  defines  as  follows : 

The  science  of  improving  stock,  which  is  by  no  means  confined 
to  questions  of  judicious  mating  but  which,  especially  in  the  case 
of  man,  takes  cognizance  of  all  influences  that  tend  in  however 
remote  a  degree  to  give  to  the  more  suitable  races  or  strains  of 


Control  of  Heredity:  Eugenics  297 

blood  a  better  chance  of  prevailing  speedily  over  the  less  suitable 
than  they  otherwise  would  have  had.  ("Inquiries  into  Human 
Faculty.") 

Fortunately  or  unfortunately  the  methods  which  breeders  use 
cannot  be  rigidly  applied  in  the  case  of  man.  It  is  possible  for 
breeders  to  eliminate  from  reproduction  all  except  the  very  best 
stocks,  and  this  is  really  essential  if  evolution  is  to  be  guided  in 
a  definite  direction.  If  only  the  very  worst  are  eliminated  in  each 
generation,  the  standard  of  a  race  is  merely  maintained,  but  the 
more  severe  the  elimination  is  the  more  does  it  become  a  directing 
factor  in  evolution.  In  the  case  of  man,  however,  even  the  most 
enthusiastic  eugenicists  have  never  proposed  to  cut  off  from  the 
possibility  of  reproduction  all  human  stocks  except  the  very  best, 
and  if  only  the  very  worst  stocks  are  thus  eliminated,  we  must 
face  the  conclusion  that  no  very  great  improvement  can  be  ef- 
fected. It  is  impossible,  then,  to  apply  rigidly  to  man  the  methods 
of  animal  and  plant  breeders.  Society  cannot  be  expected  to 
eliminate  from  reproduction  all  but  the  very  best  lines.  The  great 
majority  of  mankind  cannot  be  expected  voluntarily  to  efface 
itself.  The  most  that  can  be  hoped  for  in  this  direction  is  that 
the  great  mediocre  majority  may  eliminate  from  reproduction  a 
very  small  minority  of  the  worst  individuals. 

Furthermore,  other  and  perhaps  even  more  serious  objections 
to  the  views  of  extreme  eugenicists  are  to  be  found  in  human 
ideals  of  morality.  Even  for  the  laudable  purpose  of  producing 
a  race  of  supermen,  mankind  will  probably  never  consent  to  be 
reduced  to  the  morality  of  the  breeding-pen  with  a  total  disregard 
of  marriage  and  monogamy.  The  geneticist  who  has  dealt  only 
with  chickens  or  rabbits  or  cattle  is  apt  to  overlook  the  vast  dif- 
ference between  controlling  reproduction  in  lower  animals  and 
in  the  case  of  man  where  restraints  must  be  self-imposed. 

Another  fundamental  difficulty  in  breeding  a  better  race  of 
men  is  to  be  found  in  a  lack  of  uniform  ideals.  A  breeder  of  do- 
mestic animals  lives  long  enough  to  develop  certain  races  and  see 


2gS  Heredity  and  Environment 

them  well  established,  but  the  devotee  of  eugenics  cannot  be  sure 
that  his  or  her  ideals  will  be  followed  in  succeeding  generations. 
The  father  of  Simon  Newcomb  is  said  to  have  walked  through 
the  length  and  breadth  of  Nova  Scotia  seeking  for  himself  a 
suitable  mate,  but  neither  he  nor  any  other  eugenicist  could  be 
sure  that  his  descendants  would  follow  a  similar  course,  and  long 
continued  selection  along  particular  lines  must  be  practiced  if 
the  race  is  to  be  permanently  improved.  Mankind  is  such  a  mon- 
grel mixture,  and  it  is  so  impracticable  to  exercise  a  strict  control 
over  the  breeding  of  men,  that  it  is  hopeless  to  expect  to  get  pure 
or  homozygous  stocks  except  with  respect  to  a  very  few  charac- 
ters and  then  only  after  long  selection. 

But  granting  all  these  difficulties  which  confront  the  eugenicist, 
there  is  no  doubt  that  something  may  be  gained  by  eliminating 
merely  the  worst  human  kinds  from  the  possibility  of  reproduc- 
tion, even  though  no  marvellous  improvement  in  the  human  race 
can  be  expected  as  a  result  of  such  a  feeble  measure. 

i.  Possible  and  Impossible  Ideals.  Supermen. — What  the  fu- 
ture evolution  of  the  human  race  may  lead  to  is  an  interesting 
speculation,  but  it  is  and  can  be  only  a  speculation.  There  is  no 
present  evidence  that  there  will  ever  be  a  higher  animal  than 
man  on  the  earth,  and  the  only  evidence  that  there  may  be  a 
higher  species  than  Homo  sapiens  is  to  be  found  in  the  fact  that 
there  have  been  lower  species  of  men  in  the  past  and  that  evolu- 
tion has  been  on  the  whole  progressive.  The  idea  that  by  the  aid 
of  that  infant  industry,  eugenics,  a  new  race  of  supermen  is  shortly 
to  be  produced  is  an  iridescent  dream,  and  the  fantastic  demand 
of  some  enthusiasts  for  changes  in  racial  fashions  has  served  to 
bring  this  whole  subject  of  eugenics  into  disrepute  among 
thoughtful  men. 

Hereditary  Classes. — To  a  considerable  extent  ideals  re- 
garding individuals  and  society  have  differed  among  different 
races  in  the  past,  but  with  the  closer  communications  which  have 
been  established  between  all  parts  of  the  earth  in  modern  times 


Control  of  Heredity:  Eugenics  299 

there  has  developed  a  greater  uniformity  of  ideal.  In  a  complex 
society  all  types  of  service  are  needed  and  different  types  of  in- 
dividuals are  socially  useful.  If  the  social  good  were  the  su- 
preme end,  as  it  is  in  a  colony  of  ants  or  bees,  the  greatest  dif- 
ferentiation of  individuals  for  particular  kinds  of  service  would 
be  desirable.  There  should  be  an  hereditary  class  of  laborers,  of 
business  men,  of  scholars,  of  artists,  etc.,  and  for  the  improve- 
ment of  each  class  there  should  be  inbreeding  in  that  class.  Such 
methods  are  now  used  by  breeders  of  various  races  of  domestic 
animals  and  cultivated  plants  with  the  best  of  results.  No  breeder 
would  think  of  trying  to  improve  draft  horses  by  crossing  with 
race  horses,  nor  of  improving  milk  cows  by  crossing  with  beef 
cattle.  In  other  countries  and  ages  the  development  of  heredi- 
tary classes  and  castes  in  human  society  has  been  tried,  and  sur- 
vivals of  it  persist  to  this  day,  but  they  are  only  vestigial  rem- 
nants of  an  old  order  which  is  everywhere  being  replaced  by  a 
new  ideal  in  which  the  good  of  the  individual  as  well  as  that  of 
society  is  the  end  desired. 

The  whole  development  of  modern  society  is  in  the  direction  of 
racial  solidarity  and  away  from  hereditary  classes.  Government, 
education  and  religion;  socialism,  syndicalism,  bolshevism  all  re- 
flect the  movement  for  individual  liberty,  fraternity  and  equality. 
The  modern  ideal  individual  is  not  the  highly  specialized  unit 
in  the  social  organism,  as  in  the  case  of  social  insects,  but  rather 
the  most  general  all-round  type  of  individual,  the  man  who  can 
when  conditions  demand  it  combine  within  himself  the  functions 
of  the  laborer,  business  man,  soldier  and  scholar.  For  such  a  gen- 
eralized type  the  methods  of  inbreeding  or  close  breeding  used  by 
the  breeder  of  thoroughbreds  are  wholly  inappropriate.  On  the 
other  hand  such  a  generalized  type  must  include  the  best  qualities 
of  many  types  and  races  and  Mendelian  inheritance  shows  how 
it  is  possible  to  sort  out  the  best  qualities  from  the  worst. 

Nowadays  one  hears  a  lot  of  high  sounding  talk  about  "human 
thoroughbreds,"  which  usually  means  that  those  who  use  this 


300  Heredity  and  Environment 

phrase  desire  to  see  certain  narrow  and  exclusive  social  classes 
perpetuated  by  close  inbreeding;  it  usually  has  no  reference  to 
good  hereditary  traits  wherever  found,  indeed  such  traits  would 
not  be  recognized  if  they  appeared  outside  of  "the  four  hundred." 
Such  talk  probably  does  neither  harm  nor  good ;  the  "social  thor- 
oughbreds" are  so  few  in  number  and  so  nearly  sterile  that  the 
mass  of  the  population  is  not  affected  by  these  exclusive  classes. 

Galton  advocated  the  segregation  and  intermarriage  of  the  most 
highly  intellectual  members  of  society,  such  as  the  prize  schol- 
ars in  the  colleges  and  universities ;  but  if  the  human  ideal  is  the 
generalized  rather  than  the  specialized  type  it  would  be  better 
if  the  prize  scholars  married  the  prize  athletes.  A  race  of  highly 
specialized  scholars  or  athletes  is  not  so  desirable  as  a  race  in 
which  these  and  other  excellences  are  well  balanced.  From  this 
point  of  view  the  person  wHo  is  voted  the  "best  all-round  man 
in  his  class"  is  nearer  the  eugenical  ideal  than  the  prize  scholar. 

No  man  can  trace  his  lineage  back  through  many  generations 
without  realizing  that  it  includes  many  hereditary  lines  differing 
greatly  in  value.  The  significance  of  sexual  reproduction  lies  in 
this  very  fact  that  it  brings  about  the  commingling  of  distinct 
lines  and  thereby  makes  every  individual  different  from  every 
other  one.  The  entire  history  of  past  evolution  testifies  to  the 
value  of  this  process,  although  it  causes  the  gardener,  the  breed- 
er, the  eugenicist  serious  trouble.  But  the  gardener  can  propa- 
gate his  choice  fruits  by  budding  and  grafting,  the  breeder  can 
for  a  time  preserves  his  choice  stock  by  close  inbreeding,  but  the 
eugenicist  cannot  shut  out  the  influence  of  foreign  blood,  and  per- 
haps it  is  well  that  he  cannot  for  if  he  could  do  so  the  progress 
of  the  race  might  soon  come  to  an  end. 

Racial  Amalgamations. — In  the  human  species  the  only  absolute 
barrier  to  the  intermingling  of  races  is  geographical  isolation. 
Every  human  race  is  fertile  with  every  other  one,  and  though 
races  and  nations  and  social  groups  may  raise  artificial  barriers 
against  interbreeding  we  know  that  these  artificial  restraints  are 


Control  of  Heredity:  Eugenics  301 

frequently  disregarded  and  that  in  the  long  run  amalgamation  does 
take  place;  and  in  general  the  further  amalgamation  progresses 
the  faster  it  goes.  In  Australia  and  New  Zealand,  after  little  more 
than  a  century's  contact  with  white  races,  there  are  about  as 
many  "half  castes"  as  there  are  full  blooded  aborigines.  In  the 
United  States  one-quarter  of  all  persons  of  African  descent  con- 
tain more  or  less  white  blood;  there  are  about  eight  million  full 
blooded  negroes  and  two  million  mulattoes,  and  during  the  past 
twenty  years  the  latter  have  increased  at  twice  the  rate  of 
the  former.  In  Jamaica,  where  there  are  about  seven  hundred 
thousand  blacks  and  fifteen  thousand  whites,  there  are  about 
fifty  thousand  mulattoes.  A  similar  condition  prevails  wherever 
different  races  occupy  the  same  country.  Even  the  Jews,  who 
were  long  supposed  to  be  a  peculiarly  separate  and  distinct  peo- 
ple, have  received  large  admixtures  of  Gentile  blood  in  every 
country  in  which  they  have  lived. 

Whether  we  want  it  or  not  hybridization  of  human  races  is 
going  on  and  will  increase.  Partition  walls  between  classes  and 
races  are  being  broken  down ;  complete  isolation  is  no  longer  pos- 
sible, and  a  gradual  intermixture  of  human  races  is  inevitable. 
We  are  in  the  grip  of  a  great  world  movement  and  we  cannot 
reverse  the  current,  but  we  may  to  a  certain  extent  direct  the 
current  into  the  more  desirable  channels. 

There  is  a  popular  belief  that  hybrid  races  are  always  inferior 
to  pure  bred  ones,  but  this  is  by  no  means  the  case.  Some  hy- 
brids are  undoubtedly  inferior  to  either  of  the  parents  but  on  the 
other  hand  some  are  vastly  superior;  only  experience  can  deter- 
mine whether  a  certain  cross  will  yield  inferior  or  superior  types. 
Society  may  well  attempt  to  prevent  those  crosses  which  produce 
inferior  stock  while  encouraging  those  which  produce  superior 
types. 

Immigration. — It  is  race  mixture  which  makes  the  problem 
of  immigration  so  serious.  Generally  immigration  is  regarded 
merely  as  an  economic  and  political  problem,  but  these  aspects  of 


302  Heredity  and  Environment 

it  are  temporary  and  insignificant  as  compared  with  its  biologi- 
cal consequences.  In  welcoming  the  immigrant  to  our  shores  we 
not  only  share  our  country  with  him  but  we  take  him  into  our 
families  and  give  to  him  our  children  or  our  children's  children 
in  marriage.  Whatever  the  present  antipathies  may  be  to  such 
racial  mixtures  we  may  rest  assured  that  in  a  few  hundred  years 
these  persons  of  foreign  race  and  blood  will  be  incorporated  in 
our  race  and  we  in  theirs.  From  the  amalgamation  of  good  races 
good  results  may  be  expected;  but  fusion  with  inferior  races, 
while  it  may  help  to  raise  the  lower  race,  is  very  apt  to  pull  the 
higher  race  down.  How  insignificant  are  considerations  of  cheap 
labor  and  rapid  development  of  natural  resources  when  compared 
with  these  biological  consequences ! 

2.  Negative  Eugenical  Measures.  Late  and  Early  Marriages. 
— Galton  said  nothing  about  sterilization  or  elimination  from  re- 
production of  less  valuable  lines  in  his  "Inquiries  into  Human 
Faculty"  which  was  first  published  in  1883.  He  proposed  no 
radical  policy  but  rather  one  which  he  thought  would  be  practical 
and  might  meet  with  general  favor.  He  suggested  a  social  pol- 
icy which  would  delay  the  age  of  marriage  among  the  weak  and 
hasten  it  among  the  vigorous,  whereas  present  social  agencies  act 
in  the  opposite  direction.  He  showed  by  statistics  that,  on  the  aver- 
age, marriage  at  the  age  of  22  would  produce  at  the  end  of  one 
century  four  times  as  many  offspring  as  marriage  at  33  and  at 
the  end  of  two  centuries  ten  times  as  many.  He  particularly  em- 
phasized the  great  harm  which  would  be  done  by  an  application 
of  the  theory  of  Malthus  among  the  better  classes.  For  the  pru- 
dent to  put  off  marriage  and  to  limit  offspring  while  the  impru- 
dent continue  to  reproduce  at  the  present  rate  would  be  to  give 
the  world  to  the  imprudent  within  a  few  centuries  at  most. 

Segregation  and  Sterilization. — His  suggestions,  which  were  at 
first  received  with  indifference  or  ridicule,  were  much  less  radical 
than  the  legal  requirements  in  many  of  our  States  today.  Public 
sentiment  has  been  greatly  aroused  on  this  question;  the  appar- 


Control  of  Heredity:  Eugenics  303 

ent  increase  in  the  number  of  defectives  and  criminals  has  seemed 
to  call  for  radical  action  and  a  flood  of  hasty  but  well  intentioned 
legislation  has  been  the  result.  We  may  confidently  expect  that 
in  a  very  short  time  the  marriage  of  the  feeble-minded,  hopelessly 
insane  or  epileptic,  the  congenitally  blind,  deaf  and  dumb,  and 
those  suffering  from  many  other  inherited  defects  which  unfit 
them  for  useful  citizenship  will  be  prohibited  by  law  in  all  the 
States.  Our  immigration  laws  already  exclude  such  aliens,  and 
the  number  of  persons  of  the  types  named  who  seek  legal  consent 
to  marry  is  not  large  so  that  it  need  not  be  expected  that  such 
laws  will  quickly  improve  the  general  population.  If  in  addition 
such  persons  are  either  segregated  or  sterilized  the  danger  of 
their  leaving  illegitimate  offspring  will  be  removed;  such  pre- 
cautions have  been  taken  in  certain  of  our  States  and  will  prob- 
ably become  general,  though  at  present  few  of  the  laws  on  this 
subject  are  strictly  enforced. 

The  study  of  heredity  shows  that  the  normal  brothers  and 
sisters,  and  even  the  more  distant  relatives,  of  affected  persons 
may  carry  a  defect  as  a  recessive  in  their  germ  plasm  and  may 
transmit  it  to  their  descendants  though  not  showing  it  themselves. 
It  will  be  more  difficult,  perhaps  an  impossible  thing,  to  apply  rig- 
idly the  principles  of  good  breeding  to  such  persons  and  to  exclude 
them  from  reproduction;  but  if  in  each  generation  those  persons 
in  whom  this  recessive  trait  appears  are  prevented  from  leaving 
offspring  the  number  o  f  persons  affected  will  gradually  grow  less, 
other  conditions  being  equal. 

But  while  such  negative,  eugenical  measures  are  wholly  com- 
mendable when  applied  to  such  defects  as  those  named,  which  are 
certainly  inherited  and  which  render  those  affected  unfit  for  citi- 
zenship, the  wholesale  sterilization  of  all  sorts  of  criminals, 
alcoholics  and  undesirables  without  determining  whether  their 
defects  are  due  to  heredity  or  to  conditions  of  development  would 
be  like  burning  down  a  house  to  get  rid  of  the  rats ;  and  the  only 
justification  which  could  be  offered  for  the  general  sterilization 


304  Heredity  and  Environment 

of  the  inmates  of  all  public  institutions,  which  is  urged  by  some 
of  our  modern  crusaders,  would  be  the  defense  which  some  per- 
sons make  for  war,  namely  that  there  are  too  many  people  and 
that  anything  which  will  prevent  the  growth  of  population  is  to  be 
welcomed. 

Effects  of  Waflr  on  Race. — Advocates  of  war  never  cease  to 
point  out  its  beneficial  effects  on  the  race, — how  it  makes  men 
strong,  courageous,  unselfish,  how  it  makes  nations  great,  power- 
ful, progressive.  There  is  no  doubt  that  war  like  any  other  great 
crisis  discovers  great  men  and  furnishes  opportunities  for  the 
development  of  great  qualities  that  might  otherwise  remain  unde- 
veloped and  unknown.  But  there  is  also  no  doubt  that  it  takes 
the  very  best  blood  of  the  nations.  Those  who  go  to  war  are  the 
young,  the  strong,  the  capable,  while  the  weak,  incompetent  and 
degenerate  are  left  behind  as  unfit  for  military  service.  If  con- 
ditions could  be  reversed  and  the  bungled  and  botched,  the  feeble- 
minded and  insane,  the  degenerate  and  debauched  could  be  put 
in  the  forefront  of  battle  some  benefit  to  the  race  might  result, 
but  no  increase  of  national  greatness  can  compensate  for  the 
awful  waste  of  the  best  thing  which  any  nation  possesses — its 
best  blood. 

Realizing  that  progress  in  evolution  has  been  won  only  through 
struggle  and  that  the  human  race  owes  much  to  the  fact  that 
man  is  by  nature  and  instinct  a  "fighting  animal"  many  persons 
who  have  recognized  the  evil  effects  of  war  have  endeavored  to 
find  some  substitute  for  modern  warfare,  which  is  no  longer 
the  wager  of  personal  combat,  but  a  y^ast  impersonal  mechanism 
of  destruction.  In  view  of  the  fact  that  "intrepidity,  contempt 
for  softness,  surrender  of  private  interests,  obedience  to  com- 
mand, must  still  remain  the  rock  upon  which  states  are  built," 
William  James  proposed,  as  a  "moral  equivalent  for  war,"  com- 
pulsory service  in  hard  and  difficult  occupations  where  dangers 
and  hardships  would  be  incentives  to  effort  and  where  struggle 


Control  of  Heredity:  Eugenics  305 

for  success  would  "inflame  the  civic  temper  as  past  history  has 
inflamed  the  military  temper." 

Professor  Cannon,  whose  work  has  demonstrated  that  the 
adrenal  glands  are  par  excellence  the  glands  of  combat  and 
virility,  and  who  recognizes  the  importance  to  the  human  race  of 
maintaining  the  functional  activity  of  these  glands,  has  proposed 
athletics  anjd  especially  international  athletic  contests,  such  as 
the  Olympic  Games,  as  a  "physical  substitute  for  warfare." 

The  eugenical  ideal  is  not  a  life  of  "peace,  perfect  peace,"  nor 
a  millennium  in  which  all  struggle  shall  cease,  but  rather  a  life  of 
adventure,  conflict  and  hard-won  success.  Inaction  and  satiety 
end  in  degeneration  and  progress  can  be  purchased  only  by  strug- 
gle. But  it  is  not  only  unnecessary,  it  is  positively  irrational,  to 
resort  to  war  to  secure  these  ends.  As  civilization  advances  more 
and  more  substitutes  are  found  for  war.  Among  these  are  not 
only  athletics  and  sports  but  also  struggles  with  natural  difficulties 
and  forces  in  the  great  warfare  which  is  being  waged  for  the 
conquest  of  nature.  Even  intellectual  and  political  contests  and 
competitions  in  skill  and  workmanship  may  to  a  great  extent 
replace  war  as  a  field  of  adventure  and  emprise. 

3.  Positive  Eugenical  Measures.- — Positive  eugenical  measures 
are  much  more  difficult  to  apply  and  are  of  more  doubtful  value 
than  are  negative  ones.  Of  course  compulsory  measures  requiring 
the  best  types  to  intermarry  and  have  children  are  out  of  the 
question  and  encouragement  and  advice  alone  are  feasible.  Giv- 
ing advice  regarding  matrimony  is  proverbially  a  hazardous  per- 
formance, and  it  is  not  much  safer  for  the  biologist  than  for 
others. 

Eugenical  Predictions  Uncertain. — With  much  more  complete 
knowledge  regarding  human  inheritance  than  we  now  possess  it 
may  be  possible  to  give  eugenical  advice  wisely,  especially 
with  respect  to  physical  characteristics  which  are  hereditarily 
simple  and  generally  of  minor  significance.  But  where  the  char- 
acter is  an  extremely  complex  one  such  as  intellectual  ability,  mor- 


306  Heredity  and  Environment 

al  rectitude,  judgment  and  poise,  which  are  the  chief  characteris- 
tics which  distinguish  the  great  man  from  his  fellows,  it  will 
probably  never  be  possible  to  predict  the  result  before  the  event. 

He  would  be  a  bold  prophet  who  would  undertake  to  predict 
the  type  of  personality  which  might  be  expected  in  the  children  of 
a  given  union.  Some  very  unpromising  stocks  have  brought 
forth  wonderful  products.  Could  anyone  have  predicted  Abraham 
Lincoln  from  a  study  of  his  ancestry?  Observe  I  say  "predict," 
and  not  "explain"  after  his  appearance.  Can  anyone  now  predict 
from  what  kind  of  ancestral  combinations  the  great  scholars, 
statesmen,  men  of  affairs  of  the  next  generation  will  come?  The 
time  may  come  when  it  will  be  possible  to  predict  what  the 
chances  are  that  the  children  of  given  parents  will  inherit  more 
or  less  than  average  intellectual  capacity,  but  since  germinal  po- 
tentiality is  transformed  into  intellectual  ability  only  as  the  result 
of  development  such  a  prediction  could  not  be  extended  to  the 
latter  unless  the  environment  as  well  as  the  heredity  were  known. 

Mankind  is  such  a  mongrel  race,  good  and  bad  qualities  are 
so  mixed  in  us,  marriage  is  such  a  lottery,  the  distribution  of  the 
germinal  units  to  the  different  germ  cells  and  the  union  of  par- 
ticular germ  cells  in  fertilization  is  so  wholly  a  matter  of  chance, 
the  influence  of  even  bad  hereditary  units  on  one  another  is  so 
unpredictably  good  or  bad  as  is  shown  in  many  hybrids,  even  the 
minor  influences  of  environment  and  education  which  escape  at- 
tention are  so  potent  in  development,  that  the  chances  were  in- 
finity to  one  against  any  one  of  us,  with  all  his  individual  charac- 
teristics, ever  coming  into  existence.  If  the  Greeks  or  Romans 
had  known  of  the  real  infinity  of  chances  through  which  every 
human  being  is  brought  to  the  light  of  day  not  only  would  they 
have  deified  Chance  but  they  would  have  made  her  the  mother 
of  gods  and  men. 

Selective  Mating. — But  granting  the  impossibility  of  predicting 
the  character  of  children  it  may  well  be  asked  if  good  general  ad- 
vice may  not  be  given  regarding  the  choosing  of  a  mate.  Many 


Control  of  Heredity:  Eugenics  -    307 

people  have  thought  so,  and  if  all  that  has  been  said  or  written 
on  this  subject  were  to  be  gathered  together  I  suppose  that  there 
would  not  or  should  not  be  room  for  it  in  all  the  libraries  of  the 
world.  It  is  generally  admitted  that  few  lines  are  wholly  free 
from  hereditary  defects  and  the  question  has  often  been  asked 
what  the  eugenical  practice  should  be  in  such  cases.  Of  course 
people  with  really  serious  hereditary  defects  should  not  have 
children.  If  the  defects  are  slight  Davenport  has  suggested  that 
they  may  either  be  disregarded  or  weakness  in  any  character  may 
be  mated  with  strength  in  that  character.  That  people  with  only 
slight  hereditary  defects  should  not  marry  at  all  is  a  counsel  of 
perfection. 

On  the  other  hand  it  would  be  a  dangerous  rule  to  propose  that 
persons  having  really  serious  hereditary  defects  should  be  mated 
with  those  who  are  strong  in  those  characters  on  the  ground  that 
in  general  strength  in  a  character  is  dominant  over  weakness.  It 
has  been  suggested  that  a  normal  man  who  marries  a  feeble- 
minded woman  would  have  only  normal  children,  since  both 
genius  and  feeble-mindedness  seem  to  be  recessive  when  mated 
with  mediocrity  or  normality.  But  in  all  such  cases  the  weakness 
is  not  neutralized  or  removed  but  merely  concealed  in  the  offspring 
and  is  therefore  the  more  dangerous.  If  a  man  chooses  to  marry 
a  feeble-minded  woman  he  at  least  does  so  with  his  eyes  open  and 
he  need  not  be  deceived.  But  the  normal  and  perhaps  capable 
children  of  such  a  union  carry  the  taint  concealed  in  their  germ- 
plasm  and  if  they  should  be  mated  with  other  normal  persons 
carrying  a  similar  taint  some  of  their  children  would  be  feeble- 
minded, and  thus  the  sins  of  the  parents  in  mating  weakness  with 
strength  would  be  visited  upon  the  children  to  the  nth  genera- 
tion. Such  a  policy  of  concealing  weakness  by  mating  it  with 
strength  is  wholly  comparable  with  the  custom  once  prevalent  of 
concealing  cases  of  contagious  diseases,  and  may  properly  be  char- 
acterized as  the  "ostrich  policy." 

After  all  in  the  choosing  of  mates  a  combination  of  instinct 


308    .  Heredity  and  Environment 

and  intelligence  is  probably  the  safest  guide.  Our  instincts,  built 
up  through  long  ages,  are  generally  adaptive  and  useful,  and  if 
they  be  guided  by  reason  the  result  is  apt  to  be  better  than  if 
either  instinct  or  reason  acts  alone.  More  need  not  be  said  on 
this  subject,  since  it  is  treated  ad  infinitum  in  works  of  fiction  and 
in  ladies'  journals. 

4.  Contributory  Eugenic al  Measures.  General  Education. — In 
addition  to  these  negative  and  positive  eugenical  measures 
many  conditions  may  be  classed  as  contributory  to  eugen- 
ics. One  of  the  most  important  of  all  contributory  measures 
is  the  general  education  of  the  people  regarding  heredity.  The 
widespread  ignorance  on  this  subject  is  profound  and  very  many 
offenders  against  the  principles  of  good  breeding  have  sinned 
through  ignorance.  Any  general  reform  must  rest  upon  enlight- 
ened public  opinion,  and  the  schools,  the  churches  and  the  press 
can  do  no  more  important  work  for  mankind  than  to  educate  the 
people,  after  they  have  been  educated  themselves,  on  this  impor- 
tant matter. 

Society  too  may  cultivate  a  proper  pride  in  good  inheritance. 
Much  of  value  would  be  accomplished  if  the  silly  pride  in  ances- 
tral wealth  or  position  or  environment  which  touched  our  fore- 
bears only  superficially  and  never  entered  into  their  germplasm, 
or  the  still  sillier  claims  of  long  descent,  in  which  we  are  all 
equal,  could  be  replaced  by  a  proper  pride  in  ancestral  heredity, 
a  pride  in  those  inherited  qualities  of  body,  mind  and  character 
which  have  made  some  families  illustrious.  A  proper  pride  in 
heredity  would  do  much  to  insure  the  perpetuation  of  a  line  and 
to  protect  it  from  admixture  with  baser  blood. 

Coeducation  versus  Monasticism- — Among  other  contributory 
measures  which  serve  to  promote  good  breeding  among  men  must 
be  reckoned  coeducation,  as  well  as  other  means  of  promoting 
good  and  early  marriages.  The  president  of  a  large  coeducational 
institution  once  said  that  if  marriages  were  made  in  heaven  he  was 
sure  that  the  Lord  had  a  branch  office  in  his  university.  I  had 


Control  of  Heredity:  Eugenics  309 

occasion  a  few  years  ago  to  investigate  the  eugenical  record  of 
a  coeducational  institution,  which  is  not  unknown  in  the  world 
of  scholarship,  and  I  found  that  about  33  per  cent  of  the  recent 
graduates  had  married  fellow  students,  that  there  had  been  no 
divorces  and  that  there  were  many  children.  There  is  no  doubt 
that  coeducation  promotes  good  and  early  marriages  and  that  it 
is  not  necessarily  inimical  to  good  scholarship  even  though  it  vio- 
lates the  spirit  of  mediaeval  monasticism.  There  was  a  time  when 
it  was  supposed  that  a  scholar  must  live  the  monkish  life  of  seclu- 
sion and  contemplation,  but  the  monasteries  are  disappearing  the 
world  over,  and  it  is  time  that  the  monastic  spirit  should  go  out 
of  the  colleges  and  universities. 

On  the  other  hand  the  colleges  exclusively  for  men  or  women 
appear  to  have  a  bad  influence  on  the  marriage  rate  and  birth  rate 
of  their  graduates.  Johnson  has  shown  that  90  per  cent  of  all  the 
women  of  the  United  States  marry  before  the  age  of  40,  but  that 
among  college  women  only  half  that  number  have  married  at  the 
same  age.  As  a  result  of  investigations  at  one  of  the  leading 
women's  colleges  he  finds  that  the  marriage  and  birth  rate  of  the 
most  brilliant  students,  who  have  been  elected  members  of  Phi 
Beta  Kappa,  is  lowest  of  all.  Cattell  says  that  a  Harvard  grad- 
uate has  on  the  average  three-fourths  of  a  son,  a  Vassar  gradu- 
ate one-half  of  a  daughter. 

At  present  early  and  fruitful  marriages  among  able  and  am- 
bitious people  are  very  unfashionable  and  are  becoming  increas- 
ingly impracticable.  If  society  has  any  regard  for  its  own  welfare 
all  this  must  be  changed.  As  Galton  has  shown,  the  race  that  mar- 
ries at  22  instead  of  33  will  possess  the  earth  in  two  or  three 
centuries.  » 

The  good  of  society  demands  that  we  reverse  our  methods  of 
putting  a  premium  upon  celibacy  among  our  most  gifted  and 
ambitious  young  men  and  women,  and  if  monastic  orders  and 
institutions  are  to  continue  they  should  be  open  only  to  those  eu- 
genically  unfit. 


3io  Heredity  and  Environment 

5.  The  Declining  Birth  Rate.  Stationary  Population  Normal. 
— Among  animals  and  plants  in  a  state  of  nature  the  number  of 
individuals  in  each  species  remains  fairly  constant  from  year  to 
year ;  that  is,  only  enough  young  are  born  and  survive  to  take  the 
places  of  mature  individuals  that  die.  But  when  a  species  is 
placed  in  new  and  favorable  conditions  it  may  for  a  while  in- 
crease at  an  amazing  rate  until  the  pressure  of  population  be- 
comes sufficient  to  reestablish  an  equilibrium  between  the  birth 
rate  and  the  death  rate.  Thus  when  the  English  sparrow  was 
introduced  into  the  United  States  it  increased  at  a  phenomenal 
rate  for  a  number  of  years,  but  now  the  number  of  individuals  in 
any  given  locality  remains  about  the  same  from  year  to  year,  the 
birth  rate  merely  compensating  for  the  death  rate.  This  equili- 
brium is  brought  about  in  the  main  by  increased  mortality,  espe- 
cially among  the  young,  though  decreasing  fecundity  may  play 
a  minor  part. 

Essentially  the  same  principles  apply  to  human  populations.  Up 
to  two  or  three  centuries  ago  the  populations  of  the  older  coun- 
tries of  the  world  were  practically  stationary.  Fecundity  was 
relatively  high  but  the  death  rate  was  also  very  high,  the  excess 
of  population  in  each  generation  being  carried  off  in  large  num- 
bers by  war,  pestilence  and  famine.  Then  owing  to  the  develop- 
ments of  science  and  industry  and  to  the  opening  up  of  new 
countries  a  period  of  remarkable  expansion  of  population  began. 
The  population  of  Europe,  which  was  about  175  millions  in  1800, 
increased  to  420  millions  in  1900,  and  this  in  spite  of  the  fact  that 
about  35  millions  migrated  from  Europe  to  new  countries  during 
this  period.  This  great  increase  in  the  population  of  Europe  was 
due  primarily  to  reduction  of  the  death  rate  since  the  birth  rate 
also  declined  slightly  during  this  period,  while  in  the  newer  coun- 
tries there  was  both  an  increase  in  the  birth  rate  and  a  decrease 
in  the  death  rate. 

It  is  perhaps  an  open  question  how  long  the  advances  of  science 
in  rendering  available  the  natural  resources  of  the  earth  may  be 


Control  of  Heredity:  Eugenics  311 

able  to  keep  pace  with  increasing  population,  but  it  is  evidently 
impossible  for  this  great  increase  in  the  population  of  the  world 
to  go  on  indefinitely ;  sooner  or  later  it  must  come  to  an  end  and 
the  population  again  become  stationary.  Already  the  birth  rate 
is  decreasing  more  rapidly  than  the  death  rate  in  all  the  western 
countries  of  Europe  and  this  movement  must  ultimately  extend  to 
all  parts  of  the  world  and  lead  to  a  checking  of  the  great  increase 
in  population  which  has  characterized  the  last  two  hundred  years. 
This  approach  to  a  stationary  population  is  both  a  normal  and  a 
desirable  thing,  for  no  one  could  wish  to  see  the  population  in- 
crease more  rapidly  than  the  supply  of  food  or  other  necessaries 
of  life;  and  of  the  two  possible  methods  of  checking  population 
few  would  hesitate  to  choose  a  decreasing  birth  rate  as  preferable 
to  an  increasing  death  rate. 

It  is  not  therefore  the  declining  birth  rate  in  the  general  popu- 
lation that  should  cause  alarm  but  rather  the  declining  birth  rate 
in  the  best  elements  of  a  population,  while  it  continues  to  increase 
or  at  the  least  remains  stationary  among  the  poorer  elements,  and 
there  is  abundant  evidence  that  this  is  just  what  is  taking  place. 
The  descendants  of  the  Puritans  and  the  Cavaliers  who  have  raised 
the  cry  for  "fewer  and  better  children"  are  already  disappearing 
and  in  a  few  centuries  at  most  will  have  given  place  to  more  fer- 
tile races  of  mankind.  Many  of  the  old  New  England  families  are 
dying  out  and  their  places  are  being  taken  by  recent  immigrants. 
The  few  exceptions  are  merely  eddies  in  the  current  that  is 
bearing  them  to  doom.  In  Massachusetts  the  birth  rate  of  the  for- 
eign born  is  twice  that  of  the  native  population  while  the  death 
rate  is  about  the  same  for  both.  The  same  is  true  of  the  older 
families  in  many  parts  bf  the  world. 

Cattell  has  made  a  statistical  study  of  the  families  of  917 
American  men  of  science  and  he  finds  that  the  average  size  of 
family  of  the  parents  of  these  men  was  4-66  children,  whereas 
the  average  size  of  family  of  these  men  is  2.22  children.  In  one 
generation  the  fertility  of  these  lines  has  been  reduced  by  more 


312  Heredity  and  Environment 

than  half.  The  causes  of  this  decline  are  chiefly  voluntary  being 
assigned  to  health,  expense  and  other  causes. 

Death  of  Families. — But  the  causes  of  sterility  are  not  only 
social  and  voluntary  ones,  which  could  be  changed  by  custom  and 
public  opinion;  there  are  also  involuntary  and  biological  causes 
of  a  deep-seated  nature.  Fahlenbeck  has  made  a  study  of  433 
noble  families  of  Sweden  which  have  become  extinct  in  the  male 
line,  and  he  shows  that  the  last  male  died  unmarried  in  45  per 
cent  of  these  families,  and  before  the  age  of  21  in  39  per  cent, 
while  the  line  ended  in  infertile  marriage  in  n  per  cent  and  in 
daughters  only  in  5  per  cent. 

The  extinction  of  families,  however,  is  often  confused  with  the 
extinction  of  family  names,  which  means  only  that  the  family 
has  died  out  in  the  direct  male  line.  Biological  inheritance  does 
not  necessarily  follow  family  names.  Owing  to  the  elimination 
of  one-half  of  the  chromosomes  in  the  formation  of  the  sex  cells 
and  the  replacing  of  these  in  fertilization  by  chromosomes  from 
another  source  it  happens  that  many  persons  bear  the  name  of 
some  progenitor  but  do  not  have  a  single  one  of  his  chromosomes 
or  inherited  traits ;  on  the  other  hand,  many  persons  who  do  not 
bear  his  name  may  have  some  of  his  chromosomes  and  traits.  As- 
suming that  there  are  48  chromosomes  in  the  human  species  and 
that  these  never  break  up  or  lose  their  identity  it  is  evident  that 
no  person  can  inherit  from  more  than  48  contemporary  ancestors 
though  he  may  be  descended  from  an  innumerable  number. 

Much  confusion  is  caused  also  by  the  expression  "hereditary 
lines,"  as  if  each  family  were  separate  and  distinct  from  all 
others.  But  this  is,  of  course,  never  true.  The  only  hereditary 
lines  which  exist  are  those  of  individual  chromosomes  or  genes 
and  these  divide  and  diverge  like  the  branches  of  a  tree.  An 
individual  containing  many  chromosomes  received  from  many 
sources  belongs  to  no  single  hereditary  line,  but  rather  to  a  net- 
work of  many  lines. 

It  has  been  said  that  if  the  birth  rate  of  the  "Mayflower"  fam- 


Control  of  Heredity:  Eugenics  313 

ilies  continues  to  decrease  at  the  present  rate  for  the  next  300 
years,  all  the  survivors  at  that  time  could  be  sent  back  in  the 
original  "Mayflower."  But  there  is  no  reason  to  suspect  that 
the  decreasing  birth  rate  will  go  on  indefinitely  at  a  constant  ratio, 
and  to  assume  that  it  will  do  so  is  merely  to  look  forward  to  the 
extinction  of  all  families,  classes,  races  and  nations  in  which  the 
birth  rate  has  been  decreasing ;  this  includes  practically  the  entire 
population  of  the  United  States  and  Western  Europe  and  it  is 
evident  that  such  a  result  while  theoretically  possible  is  not  at  all 
probable.  Considering  the  large  number  of  collateral  lines  which 
have  come  from  the  "Mayflower"  stock  and  the  enormous  num- 
ber of  individuals  who  think  they  can  trace  their  ancestry  back 
to  the  "Mayflower,"  it  is  incredible  that  all  these  should  be  re- 
duced to  a  company  no  larger  than  that  which  came  over  on  that 
famous  ship. 

Broman  points  out  that  most  noble  families  of  Europe  die  out 
(probably  the  direct  male  line  only  is  meant)  after  100  to  250 
years  and  generally  do  not  live  beyond  the  third  generation.  The 
same  is  true  of  the  families  of  great  scholars,  artists  and  states- 
men. Possibly  one  cause  of  such  declining  fertility  may  be  found 
in  too  great  brain  activity,  but  there  is  no  doubt  that  in  many  in- 
stances it  is  due  to  luxurious  living.  On  the  other  hand  bodily 
fatigue  and  simple  living  favor  fertility  in  both  ariimals  and  men. 
Wild  animals  brought  into  captivity  where  they  have  comfortable 
quarters  and  an  unwonted  abundance  of  rich  food  are  usually 
infertile ;  and  the  conditions  of  life  of  the  upper  classes  of  society 
are  almost  as  unfavorable  to  fertility  as  is  captivity  with  wild  ani- 
mals. It  is  evident  that  if  we  had  fewer  luxuries  we  could  have, 
and  could  afford  to  have,  more  children. 

But  animals  in  captivity  may  gradually  become  adapted  to  their 
new  conditions  so  as  to  become  fertile,  and  there  is  evidence  that 
man  also  may  undergo  a  slow  adaptation  in  this  regard  to  condi- 
tions of  high  civilization.  Some  royal  families  of  Europe  go  back 
six  or  eight  hundred  years,  and  in  general  if  a  family  survives 


314  Heredity  and  Environment 

the  new  conditions  of  affluence  and  luxury  for  more  than  three 
generations  it  may  become  more  or  less  adapted  to  the  new  con- 
ditions. 

Birth  Control — No  eugenical  reform  can  fail  to  take  account 
of  the  fact  that  the  decreasing  birth  rate  among  intelligent  people 
is  a  constant  menace  to  the  race.  We  need  not  "  fewer  and  better 
children"  but  more  children  of  the  better  sort  and  fewer  of  the 
worse  variety.  There  is  great  enthusiasm  today  on  the  part  of 
many  childless  reformers  for  negative  eugenical  measures;  the 
race  is  to  be  regenerated  through  sterilization  or  birth  control. 
But  unfortunately  this  reform  begins  among  those  who  because 
of  good  hereditary  traits  should  not  be  infertile.  Sterlity  is  too 
easily  acquired ;  what  is  not  so  easily  brought  about  is  the  fertility 
of  the  better  ones.  Galton  was  far  wiser  than  most  of  his  follow- 
ers for  he  realized  the  necessity  of  increasing  the  families  of  the 
better  types  as  well  as  of  decreasing  those  of  the  worse. 

What  Bernard  Shaw  regards  as  the  greatest  discovery  of  the 
nineteenth  century,  viz.,  the  means  of  artificially  limiting  the  size 
of  families,  may  prove  to  be  the  greatest  menace  to  the  human 
race.  If  it  were  applied  only  to  those  who  should  not  have  chil- 
dren or  to  those  who  should  for  various  reasons  have  only  a  few 
children  it  would  be  a  blessing  to  mankind.  But  applied  to  those 
who  could  and  should  have  many  children  it  is  no  gift  of  the  gods. 
No  one  denies  that  the  chief  motive  for  limiting  the  size  of  fam- 
ilies is  personal  comfort  and  pleasure  rather  than  the  welfare  of 
the  race.  The  argument  that  people  should  have  no  more  chil- 
dren than  they  can  rear  in  comfort  or  luxury  assumes  that  en- 
vironment is  more  important  than  heredity,  which  is  contrary  to 
all  the  biological  evidence.  In  the  breeding  of  horses  or  cattle  or 
men  heredity  is  more  potent  than  environment;  and  it  is  more 
important  for  the  welfare  of  the  race  that  children  with  good  in- 
heritance should  be  brought  into  the  world  than  that  parents 
should  live  easy  lives  and  have  no  more  children  than  they  can 
conveniently  rear  amid  all  the  comforts  of  a  luxury-loving  age. 


Control  of  Heredity:  Eugenics  315 

The  method  of  evolution  in  the  past  has  been  the  production 
of  enormous  numbers  of  individuals  and  the  elimination  of  the 
least  fit.  The  modern  method  of  improving  domestic  races  is  to 
select  for  reproduction  the  best  types  from  large  numbers  of 
individuals.  Nature  has  provided  an  almost  infinite  wealth  and 
variety  of  potential  personalities  in  human  germ  cells  but  only 
an  infinitesimal  number  ever  come  to  development.  If  -this  num- 
ber is  still  further  reduced  by  artificial  means  and  without  re- 
gard to  fitness  the  race  will  be  made  the  poorer  not  merely 
in  quantity  but  also  in  quality.  The  optimism  of  those  who 
believe  that  supermen  may  be  produced  by  artificially  limiting  the 
number  of  children  is  a  foolish  and  fatal  optimism. 

Finally  for  those  who  are  denied  the  privilege  of  parenthood 
and  upon  whom  sterility  is  forced  by  whatever  circumstances  there 
is  a  lesson  of  value  to  be  drawn  from  the  social  insects.  The  ster- 
ile members  of  a  colony  of  ants  or  bees  are  forever  denied  the  pos- 
sibility of  having  offspring  of  their  own,  but  they  become  foster 
mothers  to  the  offspring  of  the  queen.  They  tenderly  nurse,  care 
for  and  rear  the  young  of  the  colony.  There  are  many  children 
in  the  world  who  need  foster  mothers  and  fathers;  there  are 
many  men  and  women  in  the  world,  both  married  and  unmarried, 
who  need  adopted  children.  "Go  to  the  ant,  thou  sluggard ;  con- 
sider her  ways  and  be  wise." 


CHAPTER  VI 
GENETICS  AND  ETHICS 


CHAPTER  VI 


GENETICS  AND  ETHICS* 

Modern  studies  of  development  are  profoundly  changing  the 
opinions  of  men  with  respect  to  human  personality.  Observa- 
tion of  the  relentless  laws  of  heredity,  of  the  inevitable  influences 
for  good  or  bad  of  environmental  conditions  over  which  the  indi- 
vidual has  no  control,  undoubtedly  tends  to  produce  a  sense  of 
helplessness  and  hopelessness.  What  light  is  thrown  upon  the 
great  problems  of  freedom  and  determinism,  of  responsibility  and 
irresponsibility,  of  duty  and  necessity  by  modern  studies  of  de- 
velopment? Such  questions  cannot  be  dealt  with  quantitatively 
or  experimentally,  and  they  lie  outside  the  field  of  exact  science, 
but  they  are  involved  in  all  inquiries  which  have  to  do  with  ra- 
tional and  social  beings;  they  lie  at  the  foundation  of  the  appli- 
cation of  science  to  human  welfare;  they  occupy  a  large  place  in 
the  thought  and  conduct  of  all  men. 

I.    THE  VOLUNTARISTIC  CONCEPTION  OF  NATURE  AND  OF 
HUMAN  RESPONSIBILITY 

Primitive  men  regarded  their  own  activities  and  all  phenomena 
of  nature  as  the  expression  of  will,  and  a  similar  view  has  been 
maintained  by  certain  philosophers  and  theologians  even  in  mod- 
ern times.  Nature  was  regarded  as  the  immediate  expression  of  a 
vast  will  which  creates,  rules,  builds  and  destroys  as  it  sees  fit. 
The  lightning  is  hurled  from  the  hand  of  Jove,  the  sea  is  dis- 

*  A  portion  of  this  chapter  was  given  as  the  presidential  address  before 
the  American  Society  of  Naturalists  in  January,  1913,  and  was  published 
in  Science  under  the  title  "Heredity  and  Responsibility." 

319 


320  Heredity  and  Environment 

turbed  by  angry  deities,  the  winds  are  let  loose  or  stilled,  the  earth 
trembles,  the  hills  smoke,  the  sun  and  moon  and  stars  travel  in 
their  appointed  courses  as  the  gods  will. 

In  this  primitive  view  of  nature  even  inanimate  objects  were 
supposed  to  be  endowed  with  wills  of  their  own,  and  many 
modern  men  are  sufficiently  primitive  to  kick  the  chair  over 
which  they  stumble,  or  to  swear  that  the  devil  has  gotten  into  the 
automobile.  Of  course  the  actions  of  all  animate  things  were 
held  to  be  the  result  of  choice;  the  fly  that  dances  on  your  head 
or  gets  into  the  soup  is  doing  it  to  annoy  you ;  the  cats  that  yowl, 
the  dogs  that  howl,  the  maniacs  that  screech  are  possessed  of 
devils,  evil  wills,  and  should  be  punished.  All  good  is  the  result 
of  good  will,  all  evil  of  evil  will.  Some  being,  some  volition,  is 
responsible  for  everything  that  happens.  All  nature  is  the  ex- 
pression of  big  or  little  wills,  of  good  or  bad  wills,  and  the  good 
should  be  rewarded  and  the  bad  punished. 

This  conception  of  nature  finds  its  counterpart  and  probably 
its  origin  in  similar  views  concerning  human  conduct  and  respon- 
sibility. According  to  this  belief  every  man  is  the  architect  of 
his  own  character;  the  will  is  absolutely  free;  no  taint  of  heredity 
or  necessity  rests  on  the  mind  or  soul ;  character  is  a  tabula  rasa, 
upon  which  the  self  writes  its  own  record  as  it  chooses,  and  is 
responsible  for  the  result.  Conduct  whether  good  or  bad,  benevo- 
lent or  criminal,  rational  or  irrational  rests  upon  voluntary  choice, 
and  for  such  choices  men  must  be  held  responsible.  To  a  great 
extent  this  view  of  freedom  and  responsibility  is  the  basis  of 
present  systems  of  government,  education,  ethics  and  religion. 

II.    THE  MECHANISTIC  CONCEPTION  OF  NATURE  AND  OF 
PERSONALITY 

As  contrasted  with  this  voluntaristic  view  of  nature  and  of 
man  consider  the  scientific  conception  of  nature  as  a  vast  mechan- 
ism, an  endless  chain  of  causes  and  effects.  Science  deals  with 
"the  unfailing  order  of  immortal  nature,"  with  the  universality 
of  cause  and  effect,  with  the  eternal  stability  and  inevitability  of 


Genetics  and  Ethics  321 

natural  processes.  Natural  phenomena  are  not  the  results  of  voli- 
tions big  or  little,  good  or  bad,  but  of  all  the  events  which  have 
gone  before.  To  the  man  of  science  nature  is  not  the  mere  caprice 
of  god  or  devil,  to  be  lightly  altered  for  a  child's  whim;  nature  is, 
as  Bishop  Butler  said,  that  which  is  "stated,  fixed,  settled,"  eter- 
nal process  moving  on,  the  same  yesterday,  to-day  and  forever. 

From  sands  to  stars,  from  the  immensity  of  the  universe  to 
the  minuteness  of  the  electron,  in  living  things  no  less  than  in  life- 
less ones,  science  recognizes  everywhere  the  inevitable  sequence 
of  cause  and  effect,  the  universality  of  natural  law.  Man  also  is  a 
part  of  nature,  a  part  of  the  great  mechanism  of  the  universe,  and 
all  that  he  is  and  does  is  limited  and  prescribed  by  laws  of  na- 
ture. Every  human  being  comes  into  existence  by  a  process  of 
development,  every  step  of  which  is  determined  by  antecedent 
causes. 

i.  The  Determinism  of  Heredity. — There  can  be  no  doubt  that 
the  main  characteristics  of  every  living  thing  are  unalterably  fixed 
by  heredity.  Men  differ  from  horses  or  turnips  because  of  their 
inheritance.  Our  family  traits  were  determined  by  the  heredi- 
tary constitutions  of  our  ancestors,  our  inherited  personal  traits 
by  the  hereditary  constitutions  of  our  fathers  and  mothers.  By 
the  shuffle  and  deal  of  the  hereditary  factors  in  the  formation  of 
the  germ  cells  and  by  the  chance  union  of  two  of  these  cells  in 
fertilization  our  hereditary  natures  were  forever  sealed.  Our 
anatomical,  physiological,  psychological  possibilities  were  prede- 
termined in  the  germ  cells  from  which  we  came.  All  the  main 
characteristics  of  our  personalities  were  born  with  us  and  cannot 
be  changed  except  within  relatively  narrow  limits.  "The  leopard 
cannot  change  his  spots  nor  the  Ethiopian  his  skin,"  and  "though 
thou  shouldst  bray  a  fool  in  a  mortar  with  a  pestle  yet  will  not  his 
foolishness  depart  from  him."  Race,  sex,  mental  capacity  are  de- 
termined in  the  germ  cells,  perhaps  in  the  chromosomes,  and  all 
the  possibilities  of  our  lives  were  there  fixed,  for  who  by  taking 
thought  can  add  one  chromosome,  or  even  one  determiner  to  his 
organization  ? 


322  Heredity  and  Environment 

The  thought  of  this  age  has  been  profoundly  influenced  by  such 
considerations.  We  formerly  heard  that  "all  men  were  created 
free  and  equal";  we  now  learn  that  "all  men  are  created  bound 
and  unequal."  We  were  once  taught  that  acts,  if  oft  repeated, 
become  habits,  and  that  habits  determine  character ;  hereditarians 
of  the  stricter  sort  now  teach  that  acts,  habits  and  character  were 
foreordained  from  the  foundation  of  the  family.  We  once  thought 
that  men  were  free  to  do  right  or  wrong,  and  that  they  were  re- 
sponsible for  their  deeds ;  now  we  learn  that  our  reactions  are 
predetermined  by  heredity,  and  that  we  can  no  more  control  them 
than  we  can  control  our  heart  beats.  For  ages  men  have  be- 
lieved in  the  influence  of  example,  in  the  uplift  of  high  ideals, 
in  the  power  of  an  absorbing  purpose;  for  ages  men  have  lived 
and  died  for  what  they  believed  to  be  duty  and  truth,  and  have 
received  the  homage  of  mankind;  or  they  have  lived  malevolent 
and  criminal  lives  and  have  been  despised  by  men  and  punished 
by  society.  But  if  our  reactions,  habits,  characters  are  prede- 
termined in  the  germplasm  such  men  have  deserved  neither 
praise  nor  blame.  If  personality  is  determined  by  heredity  alone 
all  teaching,  preaching,  government  is  useless;  freedom,  respon- 
sibility, duty  are  delusions;  whether  men  are  useful  or  useless 
members  of  society  depends  upon  their  inheritance,  and  the  only 
hope  for  the  race  is  in  eugenics — always  supposing  that  enough 
freedom  is  left  to  the  individual  or  to  society  to  control  the  im- 
portant function  of  choosing  a  mate. 

Already  a  few  enthusiastic  persons  have  begun  to  apply  these 
doctrines  to  practical  affairs.  We  are  told  that  children  should 
never  be  admonished  or  punished,  for  they  do  only  what  their 
natures  lead  them  to  do ;  the  nature  of  the  child  must  be  respected 
and  must  be  allowed  to  manifest  itself  in  its  own  way.  Lying 
and  stealing  will  cure  themselves  like  the  mumps,  or  they  will 
remain  incurable,  in  which  case  the  germplasm  is  to  blame  and 
nothing  could  have  been  done  anyway.  Laziness  is  due  to  in- 
heritance or  to  hookworms;  the  latter  kind  may  be  cured,  but 
not  the  former.  Thriftlessness,  alcoholism  and  uncleanness  run 


Genetics  and  Ethics  323 

in  families  and  can  be  cured  only  by  extermination.  Men  who 
prey  upon  society  were  born  with  wolfish  instincts,  and  cannot 
help  but  eat  the  lambs.  Villains,  lawbreakers,  murderers  should 
be  pitied  but  not  punished;  if  blame  attaches  to  their  deeds  it 
falls  upon  the  marriage  bureau  and  the  parents.  The  world  needs 
hospitals  and  sanatoria  and  sterilization  institutes  for  the  criminal 
and  the  vicious,  but  not  courts  and  prisons,  and  all  punishments 
should  be  visited  only  upon  the  parents  to  the  third  and  fourth 
generations. 

Do  our  studies  of  heredity  lead  us  to  any  such  radical  conclu- 
sions? If  they  do  we  must  accept  them  like  brave  men.  "Truth 
is  truth  if  it  sears  our  eyeballs."  But  when  theories  lead  to  such 
revolutionary  results  it  behooves  us  to  examine  carefully  those 
theories  to  see  if  there  is  not  somewhere  a  fundamental  flaw  in 
them. 

One  of  the  most  difficult  things  in  the  world  is  to  recognize  a 
great  truth,  to  feel  its  significance  and  yet  not  to  be  carried  away 
by  it.  Great  scientific  errors  are  frequently  due  not  so  much  to 
faulty  observations  as  to  sweeping  conclusions.  In  biology  the 
search  for  universal  laws  is  a  peculiarly  dangerous  pursuit.  In 
philosophy  great  errors  are  often  due  not  so  much  to  false  prem- 
ises as  to  supposed  logical  necessities.  As  a  test  of  truth  logic 
is  inferior  to  experience ;  its  faults  are  not  so  much  in  its  methods 
as  in  its  premises  and  applications.  For  this  reason  a  logical  chain 
has  led  many  a  man  into  the  bondage  of  error.  Truth  is  not 
usually  found  in  extremes,  in  "carrying  out  a  process  to  its 
logical  conclusions,"  but  rather  in  some  middle  course  which  is 
less  striking  but  more  judicious. 

Having  observed  that  the  main  characteristics  of  our  minds  as 
well  as  of  our  bodies  are  inherited,  it  is  easy  and  natural  to  go  fur- 
ther and  to  conclude  not  only  that  all  the  possibilities  of  our 
lives  are  marked  out  in  the  germ  but  that  all  that  will  actually 
develop  from  the  germ  is  there  determined  and- cannot  be  altered. 
There  are  many  similarities  between  such  an  extreme  view  and 
the  old  doctrine  of  preformation,  and  it  contains  a  like  absurdity. 


324  Heredity  and  Environment 

It  practically  denies  development  altogether.  If  the  germ  is  a 
closed  system  and  receives  nothing  from  without,  and  if  adult 
characteristics  are  predetermined  in  the  germ,  they  are  as  irre- 
vocably fixed  as  if  they  were  predelineated. 

At  the  opposite  extreme  is  the  old  voluntaristic  view  of  abso- 
lute freedom  and  absolute  responsibility.  This  view,  like  the  old 
epigenesis,  virtually  postulates  a  new  creation  for  each  individual. 
As  far  as  the  mind  and  soul  are  concerned  there  is  no  hereditary 
continuity  with  past  generations  and  none  with  future  ones.  But 
while  such  a  view  may  be  logically  complete  and  theologically 
satisfying,  it  is  not  scientific,  for  it  also  contradicts  the  evidence. 

The  truth  then  seems  to  lie  somewhere  between  these  two  ex- 
tremes. Our  personalities  were  not  absolutely  predetermined  in 
the  germ  cells  from  which  we  came,  and  yet  they  have  arisen 
from  those  germ  cells  and  have  been  conditioned  by  them.  When 
it  is  said  that  any  characteristic  is  predetermined  in  the  germ  cell, 
what  does  this  mean?  What  but  that  the  development  of  that 
characteristic  is  made  possible  ?  Adult  characteristics  are  poten- 
tial and  not  actual  in  the  germ,  and  their  actual  appearance  de- 
pends upon  many  complicated  reactions  of  the  germinal  units  with 
one  another  and  with  the  environment.  In  short,  our  actual  per- 
sonalities are  not  predetermined  in  the  germ  cells,  but  our  pos- 
sible personalities  are. 

2.  The  Determinism  of  Environment. — This  determinism  of 
heredity  is  matched  by  a  corresponding  determinism  of  environ- 
ment. Life  is  possible  only  within  rather  narrow  limits  of  physi- 
cal and  chemical  conditions  and  in  the  main  these  limits  are 
fixed  by  the  constitution  of  nature.  But  apart  from  these  ante- 
cedent conditions  of  life  in  general  there  are  many  minor  condi- 
tions of  environment  which  exercise  a  profound  influence  upon 
organisms,  especially  in  the  course  of  their  development.  Very 
slight  changes  in  food,  temperature,  moisture  and  atmospheric 
conditions  may  produce  great  changes  in  the  developing  organ- 
ism, and  these  conditions  are  for  the  most  part  entirely  beyond 
the  control  of  the  individual  affected. 


Genetics  and  Ethics  325 

In  all  organisms  the  potentialities  of  development  are  much 
greater  than  the  actualities.  In  many  animals  a  small  part  of 
the  body  is  capable,  when  separated  from  the  remainder,  of  pro- 
ducing a  whole  body,  though  this  potency  would  never  have  be- 
come an  actuality  except  under  the  stimulus  of  separation.  In 
like  manner  a  part  of  an  egg  may,  when  separated  from  the  re- 
mainder, give  rise  to  an  entire  animal.  By  modifying  the  con- 
ditions of  development  animals  may  be  produced  which  have  one 
eye,  many  eyes  or  no  eyes ;  animals  in  which  the  body  is  turned 
inside  out,  or  side  for  side;  animals  in  which  all  sorts  of  dislo- 
cation of  organs  have  taken  place;  arid  the  earlier  the  environ- 
mental forces  act  the  more  profound  are  the  modifications. 

But  leaving  out  of  account  all  forms  which  are  so  monstrous 
that  they  are  incapable  of  reaching  maturity  we  find  that  there  are 
left  many  variations  in  the  size  and  vigor  of  the  body  as  a  whole, 
as  well  as  of  its  parts ;  many  variations  in  the  more  or  less  per- 
fect correlation  of  these  parts  with  one  another,  which  were 
determined  by  the  conditions  of  development  rather  than  by 
heredity.  In  a  given  germ  cell  there  is  the  potency  of  any  kind 
of  organism  that  could  develop  from  that  cell  under  any  kind  of 
conditions.  The  potencies  of  development  are  much  greater  than 
the  actualities.  Anything  which  could  possibly  appeal  in  the 
course  of  development  is  potential  in  heredity  and  under  given 
conditions  of  environment  is  predetermined.  Since  the  environ- 
ment cannot  be  all  things  at  once  many  hereditary  possibilities 
must  remain  latent  or  undeveloped.  Consequently  the  results  of 
development  are  not  determined  by  heredity  alone  but  also  by 
extrinsic  causes.  Things  cannot  be  predetermined  in  heredity 
which  are  not  also  predetermined  in  environment. 

Of  all  animals  I  suppose  that  man  enjoys  the  most  extensive 
and  the  most  varied  environment,  and  its  effect  upon  his  person- 
ality is  correspondingly  great.  Of  all  animals  man  has  the  long- 
est period  of  immaturity  and  it  is  during  this  period  that  the  play 
of  environmental  stimuli  on  the  organism  is  effective  in  modify- 
ing development.  In  addition  to  the  material  environment  he 


326  Heredity  and  Environment 

lives  in  the  midst  of  intellectual,  social  and  moral  stimuli  which 
are  potent  factors  in  his  development.  By  means  of  his  power 
to  look  before  and  after,  he  lives  in  the  future  and  past  as  well  as 
in  the  present;  through  tradition  and  history  he  becomes  an  heir 
of  all  the  ages.  The  modifying  influences  of  all  these  environ- 
mental conditions  on  personality  are  very  great.  Each  of  us  may 
say  with  Ulysses :  "I  am  a  part  of  all  that  I  have  met."  So  great 
is  the  power  of  environment  on  the  development  of  personality 
that  it  may  outweigh  inheritance;  a  relatively  poor  inheritance 
with  excellent  environmental  conditions  often  produces  better 
results  than  a  good  inheritance  with  poor  conditions.  Of  course 
no  sort  of  environment  can  do  more  than  bring  out  the  hereditary 
possibilities,  but  on  the  other  hand  those  possibilities  must  re- 
main latent  and  undeveloped  unless  they  are  stimulated  into 
activity  by  the  environment. 

Functional  activity  or  use  is  one  of  the  most  important  factors 
of  development.  Functional  activity  is  response  to  stimuli,  which 
may  be  external  or  internal  in  origin.  The  entire  process  of  de- 
velopment may  be  regarded  as  an  almost  endless  series  of  such  re- 
sponses on  the  part  of  the  organism,  whether  germ  cell,  embryo  or 
adult,  to  external  and  internal  stimuli.  It  is  a  truism  that  use 
strengthens  a  part  and  disuse  weakens  it;  it  is  likewise  a  truism 
that  responses  which  are  oft  repeated  become  more  rapid  and 
more  perfect,  and  in  this  way  habits  are  formed.  Practically  all 
education,  whether  of  man  or  of  lower  animals,  consists  in  habit 
formation,  in  establishing  constant  relations  between  certain 
external  or  internal  stimuli  and  certain  responses  of  the  organism. 
At  first  these  stimuli  are  largely  of  external  origin ;  later  the  ex- 
ternal stimuli  may  be  replaced  more  and  more  by  internal  ones ; 
but  whatever  the  source  of  the  stimulus  the  response  of  the  or- 
ganism to  these  stimuli  is  one  of  the  most  important  factors  of 
development,  whether  of  the  body  or  of  the  mind. 

The  influence  of  environment  upon  the  minds  and  morals  of  men 
is  especially  great.  To  a  large  extent  our  habits,  words,  thoughts  ; 
our  aspirations,  ideals,  satisfactions;  our  responsibility,  morality, 


Genetics  and  Ethics  327 

religion  are  the  results  of  the  environment  and  education  of  our 
early  years.  It  cannot  be  doubted  that  if  we  had  been  born  in 
other  countries  or  ages  we  should  have  been  different  from  our 
present  selves  in  many  important  respects ;  if  we  had  been  born 
and  reared  in  the  slums  of  great  cities  we  should  have  been  other 
than  we  are;  indeed  if  the  little  illnesses,  accidents  and  contin- 
gencies of  our  lives  had  been  different  we  should  have  been  dif- 
ferent in  our  bodies  and  minds,  as  identical  twins  come  to  differ 
from  each  other  under  such  circumstances.  The  conditions  of 
early  life  and  education  have  a  great  influence  in  shaping  person- 
ality and  are  almost  as  much  beyond  the  control  of  the  individual 
as  is  heredity. 

If  personality  in  all  of  its  main  features  is  fixed  by  heredity  and 
environment  over  which  the  individual  has  little  or  no  control, 
and  this  is  certainly  true,  personality  is  as  inevitably  determined 
by  its  antecedents  as  is  any  other  natural  phenomenon.  This  is, 
I  believe,  a  conclusion  from  which  there  is  no  escape.  How  then 
is  it  possible  to  believe  in  freedom  and  responsibility?  Is  there 
not  justification  for  the  view  so  often  expressed  of  late  that  man 
is  never  free  and  that  responsibility  and  duty  are  mere  delusions  ? 

III.  DETERMINISM  AND  RESPONSIBILITY 
Many  persons  who  have  thought  upon  these  subjects  have  felt, 
apparently,  that  there  was  no  tenable  middle  ground  between  ex- 
treme voluntarism  and  extreme  mechanism;  man  has  been  re- 
garded as  a  "free  agent"  or  a  mere  "automaton,"  absolutely  free 
or  absolutely  bound,  wholly  indeterminate  or  wholly  predeter- 
mined. But  these  extreme  views  are  unreal,  unscientific  and  un- 
justifiable, for  they  contradict  the  facts  of  experience.  We  have 
the  assurance  of  experience  that  we  are  not  absolutely  free  nor 
absolutely  bound,  but  that  we  are  partly  free  and  partly  bound; 
the  alternatives  are  not  merely  freedom  or  determinism,  but 
rather  freedom  and  determinism. 

i.  Determinism  not  Fatalism. — Whatever  the  philosophical 
meaning  of  "determinism"  may  be,  all  that  is  meant  by  that  term 


328  Heredity  and  Environment 

in  science  and  in  actual  life  is  that  every  effect  is  the  resultant  of 
antecedent  causes  and  that  identical  causes  yield  identical  results. 
Determinism  does  not  mean  predeterminism :  the  one  finds  every 
effect  to  be  due  to  a  long  chain  of  preceding  causes,  the  other  at- 
tributes every  effect  to  a  single  original  cause;  the  one  is  scien- 
tific naturalism,  the  other  is  fatalism. 

Applying  this  to  personality  actual  experience  teaches  that 
constant  conditions  of  heredity  and  environment  give  constant 
results  in  development  and  that  different  conditions  give  differ- 
ent results.  Undoubtedly  the  entire  personality,  body  and  mind, 
undergoes  development,  and  modifications  of  either  heredity  or 
environment  modify  personality.  This  is  scientific  determinism, 
but  it  is  not  fatalism  and  it  is  not  incompatible  with  a  certain 
amount  of  freedom  and  responsibility. 

2.  Control  of  Phenomena  and  of  Self. — Even  the  most  extreme 
mechanists,  who  maintain  that  .we  are  mere  automata  and  that 
we  could  never  do  otherwise  than  we  do,  admit  the  possibility  of 
a  certain  amount  of  control  over  phenomena  outside  ourselves. 
They  tell  us  that  the  aim  of  science  is  not  merely  to  understand 
but  also  to  control  nature.  But  if  man  may  to  a  limited  extent 
control  physical,  chemical  and  biological  processes  in  the  world 
around  him,  if  he  may  control  to  a  limited  extent  the  behavior  of 
a  star-fish  or  dog  or  child,  on  what  ground  is  it  possible  to  deny 
a  similar  control  of  his  own  behavior?  Does  it  not  come  to  this 
that  all  such  control  means  intelligent  action,  or  rather  the  intro- 
duction of  intelligence  as  a  factor  in  the  chain  of  cause  and  effect  ? 
Before  the  appearance  of  intelligence,  whether  in  ontogeny  or 
in  phylogeny,  no  such  control  of  phenomena  or  of  self  is  possible, 
but  when  intelligence  becomes  a  factor  in  behavior  a  limited 
control  of  the  world  and  of  the  self  is  made  possible. 

Of  course  man  has  no  control  over  events  which  have  already 
happened.  Our  heredity  and  early  development  are  accomplished 
facts  which  nothing  can  change.  Development  is  not  a  reversible 
process ;  a  man  cannot  enter  a  second  time  into  his  mother's  womb 
and  be  born  again.  Once  the  sex  cells  are  formed  their  heredi- 


Genetics  and  Ethics  329 

tary  nature  is  determined ;  once  the  egg  is  fertilized  the  hereditary 
possibilities  of  the  new  individual  are  fixed;  once  any  stage  of 
development  has  passed,  that  page  in  the  book  of  life  is  closed 
and  sealed. 

And  yet  at  every  step  in  this  long  process  of  development  there 
were  one  or  more  alternatives  which  might  have  been  taken  in- 
stead of  the  one  which  was  taken.  There  were  innumerable 
possible  alternatives  in  the  matings  of  our  ancestors,  there  were 
billions  of  possible  alternatives  in  the  union  of  the  millions  of 
types  of  germ  cells  which  each  of  our  parents  produced ;  at  every 
step  in  the  development  of  the  oosperm  from  which  each  of  us 
came  there  were  many  possible  alternative  stimuli  and  responses. 
But  in  each  case  one  of  these  innumerable  alternatives  was  taken 
and  the  others  left.  In  every  instance  there  was  some  cause  that 
determined  which  alternative  was  taken,  but  these  causes  are  so 
local  and  individual  that  they  cannot  be  generalized;  one  cause 
works  in  one  instance,  another  in  another,  and  so  we  say  that 
chance  determines  which  alternative  is  taken,  meaning  by  chance 
only  this  that  the  causes  involved  cannot  be  generalized.  At 
critical  stages  in  this  process  of  development  the  alternatives  are 
so  evenly  balanced  that  minor  considerations,  which  we  call 
chance,  determine  which  path  shall  be  taken;  but  there  are  no 
backward  steps  in  development  and  once  a  path  has  been  taken 
that  particular  crisis  or  turning  point  does  not  occur  again. 

Thus  each  of  us  has  wandered  through  the  maze  of  life,  chance 
usually  determining  which  path  shall  be  taken  of  the  many  which 
heredity  and  environment  offer,  until  he  has  come  to  a  stage 
where  associative  memory  makes  it  possible  to  profit  by  exper- 
ience and  where  intellect  and  will  make  possible  intelligent  choice. 
With  the  growth  of  intellect  and  will  there  comes  to  be  a  limited 
degree  of  freedom  and  responsibility,  and  with  increasing  com- 
plexity of  organization  the  number  of  alternative  paths  is  greatly 
increased.  The  possible  reactions  of  germ  cells  are  relatively 
few  and  fixed,  the  possible  reactions  of  a  complex  animal  are  rela- 
tively many  and  behavior  is  more  plastic ;  and  thus  this  very  com- 


33°  Heredity  and  Environment 

plexity  and  plasticity  allow  adaptations  to  the  minutest  alterations 
of  environment. 

3.  Birth  and  Growth  of  Freedom. — In  animals  below  man  and 
in  the  stages  of  human  development  one  may  trace  the  birth  and 
growth  of  freedom.  Even  in  some  of  the  simplest  organisms 
one  can  observe  inhibitions  of  responses  and  modifications  of 
behavior  which  seem  to  be  due  to  conflicting  stimuli  or  to  changes 
in  the  physiological  state.  In  higher  organisms  such  inhibitions 
or  modifications  proceed  particularly  from  internal  stimuli,  which 
in  turn  are  probably  conditioned  by  hereditary  constitution  and 
past  experience.  The  factors  which  determine  behavior  are  not 
merely  the  present  stimulus  and  the  hereditary  constitution,  but 
also  the  experiences  through  which  the  organism  has  passed  and 
the  habits  which  it  has  formed. 

A  moth  cannot  avoid  the  impulse  to  fly  toward  the  light,  and  it 
does  not  learn  by  experience  to  avoid  the  flame.  Its  reactions  are 
relatively  fixed  and  machine-like.  Many  other  animals  learn  by 
experience  to  inhibit  responses  to  certain  stimuli;  a  tame  fish  or 
frog  will  take  food  from  your  hand,  but  if  it  is  repeatedly 
frightened  when  it  attempts  to  take  food  it  will  not  come  near 
you  though  it  is  starving, — it  inhibits  the  strong  impulse  of  a  hun- 
gry animal  to  take  food  by  the  counter  impulse  of  unpleasant 
memories  or  of  fear.  Here  we  have  the  beginnings  of  what  we 
call  freedom,  the  immediate  response  to  a  stimulus  is  suppressed, 
internal  stimuli  are  balanced  against  external  ones  and  final  action 
is  determined  largely  by  past  experience.  Owing  to  his  vastly 
greater  power  of  memory,  reflection  and  inhibition  man  is  much 
freer  than  any  other  animal.  Animals  which  learn  little  from 
experience  have  little  freedom  and  the  more  they  learn  the  freer 
they  become. 

In  both  ontogeny  and  phylogeny  there  has  been  development 
of  freedom.  The  reactions  of  germ  cells  and  of  the  lowest 
organisms  are  relatively  fixed.  In  more  complex  organisms  re- 
actions become  modifiable  through  conflicting  stimuli,  intelligence, 
inhibitions.  Freedom  is  the  more  or  less  limited  capacity  of  the 


Genetics  and  Ethics  331 

highest  organisms  to  inhibit  instinctive  and  rton-rational  acts  by 
intellectual  and  rational  stimuli  and  to  regulate  behavior  m  the 
light  of  past  experience.  Such  freedom  is  not  uncaused  activity, 
but  freedom  from  the  mechanical  responses  to  external  or  instinc- 
tive stimuli,  through  the  intervention  of  internal  stimuli  due  to 
experience  and  intelligence.  To  the  person  accustomed  to  think 
of  will  and  choice  as  absolutely  free  this  may  seem  to  be  a  sort  of 
freedom  so  limited  as  to  be  scarcely  worth  the  having;  and  yet 
"it  is  the  dawning  grace  of  a  new  dispensation,"  the  beginnings 
of  rational  life,  social  obligations,  moral  responsibility. 

The  only  control  over  natural  phenomena  which  is  possible  is 
in  choosing  between  alternatives  which  are  offered ;  and  the  only 
control  which  one  who  has  reached  the  age  of  intelligence  can 
have  over  his  own  development  consists  in  choosing  between  the 
alternatives  which  are  open  to  him.  He  may  not  choose  his  hered- 
ity or  early  development  for  the  alternative  paths  which  were 
once  offered  here  have  long  since  been  passed;  but  to  a  limited 
extent  he  may  choose  his  present  environment  and  training,  he 
may  choose  a  path  which  leads  to  discipline  and  increased  pow- 
ers of  self-control  or  the  reverse,  and  to  this  extent  only  is  he 
responsible  for  what  he  may  become. 

4.  Responsibility  and  Will. — All  organisms  are  capable  of 
responding  to  chemical  and  physical  stimuli  but  in  addition  normal 
men  have  the  capacity  of  responding  to  stimuli  of  a  higher  order. 
By  responsibility  I  understand  the  ability  on  the  part  of  the 
organism  to  respond  to  rational,  social  and  ethical  stimuli  or 
impulses  and  to  inhibit  responses  to  stimuli  of  an  opposite  na- 
ture, and  the  corresponding  expectation  on  the  part  of  others 
that  the  individual  will  so  respond.  The  psychical  stimuli 
which  influence  our  behavior  are  not  merely  remembered  exper- 
iences but  the  words,  suggestions,  admonitions,  ideas  which  come 
to  us  from  others,  as  well  as  the  almost  endless  permutations  of 
such  memories  and  suggestions  in  our  thoughts.  The  social  and 
ethical  stimuli  are  not  merely  such  as  arise  from  love  of  reward 
and  fear  of  punishment  or  the  desire  for  praise  and  the  fear  of 


332  Heredity  and  Environment 

blame  but  also  from  the  deep-seated  social  instinct  to  do  good, 
which  may  reach  the  highest  levels  of  altruism  and  self-sacrifice. 

The  higher  the  type  of  organization  the  larger  is  the  range  of 
stimuli  to  which  it  will  respond  and  the  larger  the  number  and 
kind  of  responses  which  may  be  called  forth ;  and  at  the  same  time 
the  larger  becomes  the  power  of  inhibition  of  responses  whether 
through  the  balancing  of  one  stimulus  against  another  or  from 
whatever  cause.  Human  responsibility  varies  with  the  complex- 
ity of  the  stimuli  involved  as  well  as  with  the  capacity  of  indi- 
viduals to  respond  to  those  stimuli.  A  man  might  be  quite  re- 
sponsible in  savage  society  who  would  be  quite  irresponsible 
in  civilized  communities.  In  an  infant  there  is  no  capacity 
to  respond  to  rational,  social  or  ethical  stimuli  but  with  in- 
creasing capacity  in  this  respect  comes  increasing  responsibil- 
ity. Mental  and  ethical  imbeciles,  insane  and  mentally  defective 
persons  have  a  low  capacity  for  such  responses  and  inhibitions  and 
consequently  less  is  expected  of  them.  There  are  in  different 
men  all  degrees  of  responsibility,  as  there  are  all  degrees  of  ca- 
pacity. In  one  and  the  same  individual  responsibility  varies  at 
different  times  and  under  different  circumstances;  it  rises  and 
falls,  like  the  tides,  in  every  life.  Varying  capacity  to  respond 
to  rational,  social  and  ethical  stimuli  and  to  inhibit  responses  of 
an  opposite  nature  depends  not  merely  upon  inheritance  but  also 
upon  training,  habits,  physiological  states.  The  common  opinion 
that  all  normal  men  are  equally  responsible  is  not  correct ;  in  the 
eyes  of  the  law  this  may  be  true,  because  legal  obligations  are  so 
far  below  the  capacities  of  normal  men  that  all  may  be  held  equally 
responsible  before  the  law,  though  in  reality  their  responsibilities 
are  as  varied  as  their  inheritance  or  their  training. 

Conversely  the  responsibility  of  society  to  the  individual  is  uni; 
versally  recognized.  •  Irresponsible  persons  must  be  cared  for  by 
older  or  wiser  persons  who  become  responsible  for  them ;  and  in 
general  the  responsibility  rests  upon  society  to  provide  as  favor- 
able environment  as  possible  for  all  its  members.  Experienced 
persons  can  to  a  certain  extent  choose  their  own  environment  and 


Genetics  and  Ethics  333 

thus  indirectly  control  their  responses  and  habits  but  young  chil- 
dren are  almost  if  not  quite  as  incapable  of  choosing  their  environ- 
ment as  of  choosing  their  heredity,  and  it  becomes  the  duty  of 
society  to  see  to  it  that  the  environmental  stimuli  are  such  as  to 
develop  rational,  social  and  ethical  habits  rather  than  the  reverse. 

We  need  not  think  of  the  will  as  a  deus  ex  machina,  nor  even 
as  "a  little  deity  encapsuled  in  the  brain,"  but  rather  as  the  sum 
of  all  those  psychical  processes,  such  as  memory  and  reason, 
which  regulate  behavior.  In  this  sense  the  will  is  as  free  as  the 
mind,  and  no  freer.  Indeed  the  will  is  the  mind  acting  as  internal 
stimulus,  inhibition,  regulation;  in  this  sense  the  existence  and 
power  of  will  is  no  more  to  be  doubted  than  the  existence  of  those 
other  mental  conditions  which  we  call  intellect  or  memory. 

Just  as  intellect  or  memory  may  be  trained  to  accomplish  re- 
sults which  would  have  been  impossible  to  the  untrained  mind,  so 
will  may  be  trained  to  initiate,  inhibit  or  regulate  behavior  in  a 
manner  quite  impossible  to  one  who  has  not  had  this  training. 
It  is  one  of  the  most  serious  indictments  against  modern  systems 
of  education  that  they  devote  so  much  attention  to  training  mem- 
ory and  intellect  and  so  little  attention  to  the  training  of  will 
upon  the  proper  development  of  which  so  much  depends. 

5.  Our  Unused  Talents. — Will  is  indeed  the  supreme  human 
faculty,  the  whole  mind  in  action,  the  internal  stimulus  which 
may  call  forth  all  the  capacities  and  powers.  And  yet  the  will  does 
not  directly  create  nor  even  discover  these  powers ;  they  are  pro- 
duced by  the  factors  of  development,  by  heredity,  environment 
and  training ;  and  they  are  usually  discovered  by  accident  or  under 
the  stress  of  necessity.  How  often  have  we  surprised  ourselves 
by  doing  some  unusual  or  prodigious  task !  What  we  have  once 
done  we  feel  that  we  can  do  again.  We  realize  more  or  less 
clearly,  depending  upon  our  experience,  that  what  we  habitually 
do  is  far  less  than  we  could  do.  It  is  this  reserve,  upon  which  we 
can  draw  on  special  occasions,  that  gives  us  the  sense  of  freedom. 

In  his  inspiring  address  on  "The  Energies  of  Men"  William 
James  showed  that  we  have  reservoirs  of  power  which  we  rarely 


334  Heredity  and  Environment 

tap,  great  energies  upon  which  we  seldom  draw,  and  that  we 
habitually  live  upon  a  level  which  is  far  below  that  which  we  might 
occupy.  Darwin  held  the  opinion,  as  the  result  of  a  lifetime  of 
observation,  that  men  differ  less  in  capacity  than  in  zeal  and  de- 
termination to  utilize  the  powers  which  they  have.  In  playful 
comment  on  the  variety  and  extent  of  his  own  life  work  he  said 
in  modest  and  homely  phrase,  "It's  dogged  as  does  it."  It  may 
be  objected  that  the  zeal  and  determination  were  inherited,  but 
here  also  the  hereditary  possibilities  become  actualities  only  as 
the  result  of  use,  training,  the  formation  of  habits. 

It  is  generally  admitted  that  no  constant  distinction  can  be 
recognized  between  the  brain  of  a  philosopher  and  that  of  many 
a  peasant.  Neither  size  nor  weight  of  brain  nor  complexity  of 
convolutions  bears  any  constant  relation  to  ignorance  or  intelli- 
gence, though  doubtless  an  "unlimited  microscopist"  could  find 
differences  between  the  trained  and  the  untrained  brain.  The 
brains  of  Beethoven,  Gauss  and  Cuvier,  although  unusually  large, 
have  been  matched  in  size  and  visible  complexity  by  the  brains  of 
unknown  and  unlearned  persons — persons  who  were  richly  en- 
dowed by  nature  but  who  had  never  learned  to  use  their  talents. 
In  all  men  the  capacity  for  intellectual  development  is  probably 
much  greater  than  the  actuality.  The  parable  of  the  talents  ex- 
presses a  profound  biological  truth,  men  differ  in  hereditary  en- 
dowments, one  receives  ten  talents  and  another  receives  but  one ; 
but  the  used  talent  increases  many  fold,  the  unused  remains  un- 
changed and  undeveloped.  Happy  is  he  who  is  compelled  to  use 
his  talents ;  thrice  happy  he  who  has  learned  how  to  compel  him- 
self !  We  shall  not  live  to  see  the  day  when  human  inheritance 
is  greatly  improved,  though  that  time  will  doubtless  come,  but  in 
the  meantime  we  may  console  ourselves  by  the  thought  that  we 
have  many  half -used  talents,  many  latent  capacities,  and  although 
we  may  not  be  able  to  add  to  our  inheritance  new  territory  we 
may  greatly  improve  that  which  we  have. 

Jennings  has  pointed  out  as  one  of  the  great  tragedies  of  life 
the  almost  infinite  slaughter  of  potential  personalities  in  the  form 


Genetics  and  Ethics  335 

of  germ  cells  which  never  develop.  A  more  dreadful  though 
less  universal  tragedy  is  the  loss  of  real  personalities  who  have 
all  the  native  endowments  of  genius  and  leadership  but  who  for 
lack  of  proper  environmental  stimuli  have  remained  undeveloped 
and  unknown;  the  "mute,  inglorious  Miltons"  of  the  world;  the 
Caesars,  Napoleons,  Washingtons  who  might  have  been ;  the  New- 
tons,  Darwins,  Pasteurs  who  were  ready  formed  by  nature  but 
who  never  discovered  themselves.  One  shudders  to  think  how 
narrowly  Newton  escaped  being  an  unknown  farmer,  or  Faraday 
an  obscure  bookbinder,  or  Pasteur  a  provincial  tanner.  In  the  his- 
tory of  the  world  there  must  have  been  many  men  of  equal  native 
endowments  who  missed  the  slender  chance  which  came  to  these. 
We  form  the  habit  of  thinking  of  great  men  as  having  appeared 
only  at  long  intervals,  and  yet  we  know  that  great  crises  always 
discover  great  men.  What  does  this  mean  but  that  the  men  are 
ready  formed  and  that  it  requires  only  this  extra  stimulus  to  call 
them  forth?  To  most  of  us  heredity  has  been  kind — kinder  than 
we  know.  The  possibilities  within  us  are  great  but  they  rarely 
come  to  full  epiphany. 

6.  Self  Knowledge  and  Self  Control. — What  is  needed  in 
education  more  than  anything  else  is  some  means  or  system 
which  will  train  the  powers  of  self  discovery  and  self  con- 
trol. Easy  lives  and  so-called  "good  environment"  will  not 
arouse  the  dormant  powers.  It  usually  takes  the  stress  and 
strain  of  hard  necessity  to  make  us  acquainted  with  our  hidden 
selves,  to  rouse  the  sleeping  giant  within  us.  How  often  is  it  said 
that  the  worthless  sons  of  worthy  parents  are  mysteries;  with 
the  best  of  heredity  and  environment  they  amount  to  nothing, 
whereas  the  sons  of  poor  and  ignorant  farmers,  blacksmiths, 
tanners  and  backwoodsmen,  with  few  opportunities  and  with 
many  hardships  and  disadvantages,  become  world  figures.  Prob- 
ably the  inheritance  in  these  last  named  cases  was  no  better  than 
in  the  former,  but  the  environment  was  better.  "Good  environ- 
ment" usually  means  easy,  pleasant,  refined  surroundings,  "all 
the  opportunities  that  money  can  buy,"  but  little  responsibility 


336  Heredity  and  Environment 

and  none  of  that  self  discipline  which  reveals  the  hidden  powers 
and  which  alone  should  be  counted  good  environment.  Many 
schools  and  colleges  are  making  the  same  mistake  as  the  fond 
parents;  luxury,  soft  living,  irresponsiblity  are  not  only  allowed, 
but  are  encouraged  and  endowed — and  by  such  means  it  is  hoped 
to  bring  out  that  in  men  which  can  only  be  born  in  travail. 

The  chief  educational  value  of  athletics  is  found  in  this  that 
it  teaches  self  control.  But  in  great  athletic  contests  the  self 
control  of  the  spectators  is  usually  inversely  proportional  to  that 
of  the  players,  and  while  excess  of  stimuli  may  lead  to  wholesome 
and  beneficial  reactions  in  the  players  it  frequently  leads  to  excess 
of  stimulants  and  to  other  injurious  reactions  in  the  spectators. 
But  college  athletics  has  this  much  at  least  in  its  favor,  it  trains 
men  who  take  part  in  the  contests  to  do  their  best,  to  subordinate 
pleasure,  appetite,  the  desire  for  a  good  time,  to  one  controlling 
purpose ;  it  trains  them  to  attempt;  what  may  often  seem  to  them 
impossible,  to  crash  into  the  line  though  it  may  seem  a  stone  wall, 
to  get  out  of  their  bodies  every  ounce  of  strength  and  endurance 
which  they  possess.  Such  training  makes  men  acquainted  with 
their  powers  and  teaches  courage,  confidence  and  responsibility. 
If  only  we  could  make  young  persons  acquainted  in  some  similar 
way  with  their  hidden  mental  and  moral  powers  what  a  race  of 
men  and  women  might  we  not  have  without  waiting  for  that  un- 
certain day  when  the  inheritance  of  the  race  will  be  improved! 
Whatever  the  stimulus  required,  whether  pride  or  shame,  fear 
or  favor,  ambition  or  loyalty,  responsibility  or  necessity,  education 
should  utilize  each  and  all  of  these  to  teach  men  self  knowledge 
and  self  control. 

But  it  will  be  said  that  self  control  depends  upon  inheritance, 
that  strong  wills  and  weak  wills  are  such  because  of  heredity. 
It  is  true  that  one  man  may  be  born  with  a  potentiality  for  self 
control  which  another  man  lacks,  but  in  all  men  this  potentiality 
becomes  actuality  only  through  development,  one  of  the  principal 
factors  of  which  is  use  or  functional  activity.  An  amazing  num- 
ber of  persons  have  but  little  self  control.  Is  this  always  due  to 


Genetics  and  Ethics  337 

defective  inheritance,  or  is  it  not  frequently  the  result  of  bad 
habits,  of  arrested  development?  To  charge  defects  at  once  to 
heredity  removes  them  from  any  possible  control,  helps  to  make 
men  irresponsible,  excuses  them  for  making  the  least  of  their 
endowments.  To  hold  that  everything  has  been  predetermined, 
that  nothing  is  self  determined,  that  all  our  traits  and  acts  are 
fixed  beyond  the  possibility  of  change  is  an  enervating  philosophy 
and  is  not  good  science,  for  it  does  not  accord  with  the  evidence. 
It  is  amazing  that  men  whose  daily  lives  contradict  this  paralyzing 
philosophy  still  hold  it,  as  it  were  in  some  water-tight  compart- 
ment of  the  brain,  while  in  all  the  other  parts  of  their  being  their 
acts  proclaim  that  they  believe  in  their  powers  of  self  control: 
they  set  themselves  hard  tasks,  they  overcome  great  difficulties, 
they  work  until  it  hurts,  until  they  can  say  with  Johannes  Miiller, 
Es  klebt  Blut  an  der  Arbeit,  and  yet  in  the  philosophical  com- 
partment of  their  minds  they  can  say  that  it  was  all  predetermined 
in  heredity  and  from  the  foundations  of  the  world. 

Whether  all  the  phenomena  of  life  and  of  mind  can  be  explained 
on  the  basis  of  a  purely  mechanistic  hypothesis  or  not,  that  hy- 
pothesis must  square  with  the  facts  and  not  the  facts  with  the 
hypothesis.  It  has  always  been  true  of  those  who  "sat  apart  and 
reasoned  high  of  fate,  free  will,  foreknowledge  absolute'*  that 
they  have  "found  no  end  in  wandering  mazes  lost."  Whatever 
the  way  out  of  these  mazes  may  be, — whether  it  be  found  in  the 
varied  responses  of  an  organism  to  the  same  stimulus,  to  the  intro- 
duction of  memory,  intelligence  and  reason  as  internal  stimuli, 
or  to  some  form  of  idealism  which  finds  necessity  not  in  nature 
but  in  the  spectator,  and  freedom  not  in  the  spectator  but  in  the 
agent, — it  is  true  for  those  who  do  not  "sit  apart  and  reason 
high,"  but  who  deal  merely  with  evident  phenomena,  that  the  way 
of  escape  is  not  to  be  found  in  denying  the  reality  of  inhibition, 
will  and  self  control.  Because  we  can  find  no  place  in  our  phi- 
losophy and  logic  for  self  determination  shall  we  cease  to  be 
scientists  and  close  our  eyes  to  the  evidence?  The  first  duty  of 
science  is  to  appeal  to  fact  and  to  settle  later  with  logic  and 


338  Heredity  and  Environment 

philosophy.  Is  it  not  a  fact  that  the  possibilities  of  our  inheritance 
depend  for  their  realization  upon  development,  one  of  the  most 
important  factors  of  which  is  use,  functional  activity  in  response 
to  stimuli?  Is  it  not  a  fact  that  in  many  animals  behavior  is 
modifiable  and  that  impulses  may  be  inhibited  and  controlled  ?  Is 
it  not  a  fact  that  experience,  intelligence,  will  are  factors  in  hu- 
man behavior  and  that  by  means  of  these  men  are  often  able  to 
choose  between  alternatives  and  so  to  control  their  own  activi- 
ties as  well  as  external  phenomena?  Is  it  not  a  fact  that  our  ca- 
pacities are  very  much  greater  than  our  habitual  demands  upon 
them?  Is  it  not  a  fact  that  belief  in  our  responsibility  energizes 
our  lives  and  gives  vigor  to  our  mental  and  moral  fiber?  Is  it  not 
a  fact  that  shifting  all  responsibility  from  men  to  their  heredity 
or  to  that  part  of  their  environment  which  is  beyond  their  con- 
trol helps  to  make  them  irresponsible? 

This  debilitating  philosophy  in  which  everything  is  predeter- 
mined, in  which  there  is  no  possibility  of  change  or  control,  in 
which  there  is  hypertrophy  of  intellect  and  atrophy  of  will,  is  a 
symptom  of  senility  whether  in  men  or  nations.  We  need  to  re- 
turn to  the  joys  of  a  childhood  age  in  which  men  believed  them- 
selves free  to  do,  to  think,  to  strive,  in  which  life  was  full  of 
high  endeavor  and  the  world  was  crowded  with  great  emprise. 
We  need  to  think  of  the  possibilities  of  development  as  well  as  of 
the  limitations  of  heredity.  Chance,  heredity,  environment  have 
settled  many  things  for  us;  we  are  hedged  about  by  bounds 
which  we  cannot  pass,  but  those  bounds  are  not  so  narrow  as  we 
are  sometimes  taught  and  within  them  we  have  a  considerable 
degree  of  freedom  and  responsibility. 

"That  which  we  are  we  are, 
One  equal  temper  of  heroic  hearts 
Made  weak  by  time  and  fate,  but  strong  in  will 
To  strive,  to  seek,  to  find,  and  not  to  yield." 


Genetics  and  Ethics  339 

IV.  THE  INDIVIDUAL  AND  THE  RACE 
There  is  a  larger  freedom  and  a  greater  responsibility  than 
that  which  characterizes  the  individual.  What  the  individual 
cannot  do  because  of  weakness,  ignorance,  self  interest,  short 
life,  society  can  accomplish  with  the  strength,  wisdom  and  in- 
terest of  all,  and  through  long  periods  of  time.  There  are  many 
grades  of  organization  from  the  bacterium  to  the  vertebrate, 
from  the  germ  cell  to  the  man.  Society  is  the  last  and  highest 
grade  of  organization  and  its  freedom  and  responsibility  are  to 
those  of  the  individual  very  much  as  the  freedom  and  responsi- 
bility of  the  developed  man  are  to  those  of  the  germ  cell  from 
which  he  came.  Out  of  the  correlations,  differentiations  and 
integrations  of  persons  has  grown  this  higher  type  of  organiza- 
tion which  we  call  society. 

i.  The  Conflict  between  the  Freedom  ^of  the  Individual  and 
the  Good  of  Society. — The  freedom,  power  and  responsibility  of 
society  are  founded  upon  limitations  of  individual  freedom  for 
the  good  of  the  race.  Among  social  animals,  such  as  ants  and 
bees,  there  is  so  much  instinct  and  so  little  reason  and  freedom 
that  there  is  practically  no  conflict  between  the  individual  and 
the  race,  but  with  the  increase  of  intelligence  and  freedom  among 
men  there  has  developed  an  increasing  conflict  between  the  indi- 
vidual and  society.  So  far  as  social  limitations  are  artificial,  sel- 
fish, for  the  good  of  a  few  rather  than  of  all,  this  conflict  of  the 
ages,  this  struggle  to  be  free  has  been  the  crowning  glory  of  man- 
kind. The  struggle  for  freedom  from  tyranny  in  thought  and 
speech,  in  religion,  government  and  industry,  no  less  than  for 
the  freedom  that  comes  by  the  conquest  of  nature,  is  one  of  the 
greatest  achievements  of  the  human  race. 

But  social  restrictions  on  individual  freedom  are  not  all  artifi- 
cial and  selfish.  Some  of  them  are  absolutely  essential  not  only  to 
the  welfare  but  even  to  the  continued  existence  of  the  race,  and 
when  demands  for  individual  freedom  go  to  the  extent  of  fight- 
ing against  these  racial  obligations  they  become  a  serious  men- 
ace to  mankind. 


34°  Heredity  and  Environment 

2.  Perpetuation  and  Improvement  of  the  Species  the  Highest 
Ethical  Obligation. — Among  all  organisms  the  race  or  species  is 
of  paramount  importance.  Race  preservation,  not  self  preserva- 
tion, is  the  first  law  of  nature.  Among  all  organisms  the  perpetu- 
ation and  welfare  of  the  race  are  cared  for  by  the  strongest  in- 
stincts. In  very  many  species  of  animals  reproduction  means  the 
death  of  the  individual.  The  breeding  instinct  drives  every  male 
bee,  every  male  and  female  salmon,  to  its  certain  death  in  order 
that  the  race  may  be  perpetuated.  Among  the  higher  organisms 
the  strongest  of  all  the  instincts  are  those  connected  with  repro- 
duction. But  in  the  human  species  intellect  and  freedom  come  in 
to  interfere  with  instinct.  The  reproductive  instincts  are  not 
merely  controlled  by  reason,  as  they  should  be,  but  to  an  alarming 
extent  they  are  thwarted  and  perverted  among  intelligent  people. 

The  struggle  to  be  free  is  part  of  a  great  evolutionary  move- 
ment, but  the  freedom  must  be  a  sane  one  which  neither  injures 
others  nor  eliminates  posterity.  The  feminist  movement  in  so 
far  as  it  demands  greater  intellectual  and  political  freedom  for 
women  may  be  a  benefit  to  the  race  but  in  so  far  as  it  demands 
freedom  from  marriage  and  reproduction  it  is  suicidal.  The  cry 
of  Rachel,  "Give  me  children  or  I  die,"  has  been  turned  by  many 
modern  women  to,  "I'd  rather  die  than  have  children."  If  the 
demand  for  individual  freedom  blinds  men  and  women  to  their 
racial  obligations  the  inevitable  decadence  and  extinction  of  their 
lines  must  follow.  In  every  age  and  country  where  demands  for 
personal  freedom  have  been  most  insistent  and  extreme,  where 
men  and  especially  women  have  demanded  freedom  from  the  bur- 
dens of  bearing  and  rearing  children  as  well  as  from  other  natural 
social  obligations,  the  end  has  been  degeneration  and  extinction. 

This  has  been  the  history  of  many  talented  races  and  families 
of  mankind.  The  decay  of  the  most  gifted  races  of  the  ancient 
world,  especially  those  of  Greece  and  Rome,  was  not  due  pri- 
marily to  bad  heredity  nor  to  bad  material  environment  but  rather 
to  the  growth  of  luxury  and  selfishness  and  unrestricted  free- 
dom; marriage  became  unfashionable,  immorality  was  wide- 


Genetics  and  Ethics  341 

spread,  and  then  came  sterility  and  extinction  or  mixture  with 
inferior  stock  and  degeneracy.  And  then  the  barbarian,  the  im- 
migrant, the  natural  man,  unspoiled  by  too  much  freedom  and 
true  to  his  instincts,  came  in  to  take  the  place  of  the  more  gifted 
race.  Truly  "there  is  a  power  not  ourselves  that  makes  for 
righteousness." 

In  these  days  when  we  talk  of  our  race  and  our  civilization  as 
if  they  were  necessarily  supreme  and  immortal  it  is  well  to  re- 
member that  there  have  been  other  races  and  other  civilizations 
that  regarded  themselves  in  the  same  way. 

"Assyria,  Greece,  Rome,  Carthage,  where  are  they?" 
And  what  assurance  have  we  that  our  race  and  our  civilization 
will  not  run  a  similar  course  and  come  to  a  similar  end  ?  May  we 
not  surely  predict  that  if  we  continue  to  put  individual  freedom 
and  luxury  and  selfishness  above  social  obligations  our  race  and 
civilization  will  also  see  the  writing  on  the  wall,  "Thou  art  weighed 
in  the  balances  and  art  found  wanting?"  In  these  days  when  in- 
dividuals are  demanding  more  and  more  freedom  it  is  well  to 
remember  that  "the  best  use  that  man  has  made  of  his  freedom 
has  been  to  place  limitations  upon  it."  Again  and  again,  age  after 
age,  men  and  families  and  nations  have  gone  up  to  a  climax  of 
greatness  and  then  have  declined,  while  other  unknown  men  have 
taken  their  places.  Greatness  has  not  for  long  perpetuated  itself. 
An  epitome  of  human  history  is  contained  in  the  words,  "He  hath 
put  down  the  mighty  from  their  seats  and  hath  exalted  them  of 
low  degree." 

It  may  well  be  asked  by  those  who  are  interested  in  breeding  a 
better  race  of  men  whether  such  a  thing  is  possible,  whether  the 
better  race  may  not  be  lacking  in  vitality  or  fertility  or  morality 
and  thus  be  doomed  to  an  early  end.  Although  this  has  been  the 
fate  of  many  gifted  races  of  the  past  I  do  not  think  that  it  was  a 
necessary  fate.  The  history  of  domesticated  animals  and  of  cul- 
tivated plants,  and  especially  the  recent  notable  advances  in 
genetics,  indicate  what  eugenics  might  do  for  the  human  race.  In 


342  Heredity  and  Environment 

time,  under  intelligent  guidance,  the  worst  qualities  of  the  race 
might  be  weeded  out  and  the  best  qualities  preserved.  This  is 
the  goal  toward  which  intelligent  effort  should  be  directed.  This 
should  be  the  supreme  duty  of  society  and  of  all  who  love  their 
fellow  men. 

But  I  think  that  notable  human  improvement  can  take  place 
only  upon  two  conditions :  ( i )  The  physical  and  intellectual  im- 
provement of  the  individual  through  environment  and  training 
must  not  interfere  with  his  racial  and  ethical  obligations.  Indi- 
vidual freedom  must  be  subordinated  to  racial  welfare.  (2)  The 
promotion  of  human  evolution  must  be  undertaken  by  society  as 
its  greatest  work.  Not  only  has  society  greater  freedom  and 
greater  power  than  the  individual  but  it  persists  while  men  come 
and  go. 

Our  hereditary  lines  are  so  interwoven  with  those  of  other 
races  and  will  be  so  entangled  with  other  lines  in  the  future  that 
any  selfish  or  narrow  policy  of  improving  our  family  or  class  can 
have  little  permanent  value.  We  shall  rise  only  as  our  race  rises. 
Indeed  when  we  consider  all  the  influences  of  our  fellow  men 
upon  our  development,  when  we  consider  our  hereditary  con- 
nections with  multitudes  of  men  and  women  of  the  past,  when 
we  think  of  the  nexus  of  hereditary  strands  which  are  woven  into 
our  personalities  and  which  will  be  continued  through  us  to 
many  future  generations,  we  realize  that  after  all  the  individual  is 
not  really  a  separate  and  independent  being,  but  a  minor  unit  in 
the  great  organism  of  humanity,  and  that  his  greatest  duty  is  to 
transmit  unimpaired  and  undefiled  a  noble  heritage  to  generations 
yet  unborn. 

It  is  possible  greatly  to  improve  environment.  Conditions  of 
life  are  still  hard  and  cruel  for  many.  A  vast  amount  of  good 
human  material  is  wasted  in  modern  society.  As  civilization  be- 
comes more  complex  the  quantity  of  human  wreckage  and  wast- 
age ever  grows  greater.  Many  useful  lives  and  some  great  possi- 
bilities are  blotted  out  by  unfavorable  environment.  It  is  the 


Genetics  and  Ethics  343 

duty  of  society  as 'far  as  possible  to  conserve  these  lives  and  to 
develop  these  possibilities. 

It  is  possible  greatly  to  improve  education,  to  make  it  a  potent 
factor  in  development  instead  of  a  conventional  veneer.  In  spite 
of  innumerable  educational  reforms  the  essential  reform  has  not 
yet  been  reached;  mere  refinements  of  bad  methods  are  not  real 
reforms.  The  essence  of  all  education  is  self  discovery  and  self 
control.  When  education  helps  an  individual  to  discover  his  own 
powers  and  limitations  and  shows  him  how  to  get  out  of  his 
heredity  its  largest  and  best  possibilities  it  will  fulfil  its  real  func- 
tion; when  children  are  taught  not  merely  to  know  things  but 
particularly  to  know  themselves,  not  merely  how  to  do  things  but 
especially  how,  to  compel  themselves  to  do  things,  they  may  be 
said  to  be  really  educated.  For  this  sort  of  education  there  is 
demanded  rigorous  discipline  of  the  powers  of  observation,  of 
the  reason,  and  especially  of  the  will. 

It  is  possible  greatly  to  improve  heredity:  (a)  By  weeding  out 
from  the  possibility  of  reproduction  human  stocks  bearing  serious 
defects,  (b)  By  cultivating  pride  in  good  heredity  and  by  dis- 
couraging voluntary  infertility  on  the  part  of  those  who  have  a 
goodly  heritage,  (c)  By  increasing  opportunities  for  early  and 
favorable  marriages,  (d)  By  carefully  conserving  the  best  hu- 
man mutations  or  inherited  variations.  In  this  way  if  in  any  way 
the  better  race  will  be  produced.  The  possible  improvements  of 
heredity  are  great,  the  possible  improvements  of  environment 
and  training  are  great,  but  whether  men  of  the  future  will  be  bet- 
ter than  those  of  the  past  or  present  is  a  question  not  only  of 
genetics  but  also  of  ethics. 

How  better  can  I  close  this  course  of  lectures  than  with  the 
words  of  Francis  Galton,  one  of  the  greatest  students  of  human 
heredity  and  the  founder  of  the  science  of  Eugenics? 

"The  chief  result  of  these  inquiries  has  been  to  elicit  the  reli- 
gious significance  of  the  doctrine  of  evolution.  It  suggests  an 
alteration  in  our  mental  attitude  and  imposes  a  new  moral  duty. 


344  Heredity  and  Environment 

The  new  mental  attitude  is  one  of  a  greater  sense  of  moral  free- 
dom, responsibility  and  opportunity;  the  new  duty  which  is  sup- 
posed to  be  exercised  concurrently  with,  and  not  in  opposition  to 
the  old  ones  upon  which  the  social  fabric  depends,  is  an  en- 
deavor to  further  evolution,  especially  that  of  the  human  race." 


REFERENCES  TO  LITERATURE 

The  following  list  of  books  and  publications  includes  only  those 
works  which  are  referred  to  most  frequently  in  the  preceding 
pages.  Several  of  the  books  cited,  particularly  those  by  Plate, 
Morgan,  Babcock  and  Clausen,  contain  extensive  bibliographies. 
Those  desiring  to  become  more  fully  acquainted  with  books  and 
articles  dealing  with  the  subjects  of  heredity  and  development  are 
referred  to  the  larger  works  which  are  listed  here. 

Books  and  Larger  Works 

Babcock  and  Clausen.    Genetics  in  Relation  to  Agriculture.    New 
York,  1918. 

Bateson,  W.    Materials  for  the  Study  of  Variation,  London,  1894. 

Bateson,  W.    Problems  in  Genetics.    Yale  Univ.  Press.    1913. 

Bateson,  W.    Mendel's  Principles  of  Heredity.    3rd  Impression, 
Cambridge,  1913. 

Baur,   E.     Einfiihrung  in  die  experimentelle   Vererbungslehre. 
Berlin,  1911. 

Cannon,  W.  B.    Bodily  Changes  in  Pain,  Hunger,  Fear  and  Rage. 
New  York,  1915. 

Castle,  W.    Heredity  in  Relation  to  Evolution  and  Animal  Breed- 
ing.   New  York,  1912. 

Castle,  W.  E. ;  Coulter,  J.  M. ;  Davenport,  C.  B. ;  East,  E.  M. ; 
Tower,  W.  L.    Heredity  and  Eugenics.    Chicago,  1912. 

Correns,  C.    Die  Neuen  Vererbungsgesetze.    Berlin,  1912. 

Darbishire,  A.  R.     Breeding  and  Mendelian  Discovery,  Cotton. 
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Darwin,  C.     Animals  and  Plants  under  Domestication.     New 
York,  1887. 

Davenport,  C.  B.    Heredity  in  Relation  to  Eugenics.    New  York, 
1911. 

De  Vries,  H.     Intracellular  Pangenesis.     Jena,  1889. 

De  Vries,  H.    Die  Mutationstheorie.    Leipzig,  1901. 

De  Vries,  H.    Plant  Breeding.     Chicago,  1907. 

Doncaster,  L.    Heredity  in  the  Light  of  Recent  Research.    Cam- 
bridge, 1911. 

Driesch,  H.    The  Science  and  Philosophy  of  the  Organism.    Gif- 
ford  Lectures,  London,  1908. 

345 


346  Heredity  and  Environment 

Ellis,  H.    The  Task  of  Social  Hygiene.    London,  1912. 

Ellis,  H.    The  Problem  of  Race  Regeneration.    New  York,  1911. 

Forel,  A.    The  Sexual  Question.    New  York,  1908. 

Galton,  F.    Inquiries  into  Human  Faculty.    New  York,  1883. 

Galton,  F.     Natural  Inheritance.     London,  1889. 

Galton,  F.    Hereditary  Genius.    London,  1892. 

Galton,  F.     Essays  in  Eugenics.     London,  1909. 

Goddard,  H.  H.     The  "Kallikak"  Family.     New  York,  1912. 

Goldschmidt,    R.      Einfuhrung   in   die   Vererbungswissenschaft. 

Leipzig,  1911. 

Hacker,  V.    Allgemeine  Vererbungslehre.     Braunschweig,  1912. 
Hertwig,  O.    Allgemeine  Biologic.    Jena,  1909. 
Johannsen,  W.    Elemente  der  exakten  Erblichkeitslehre,  2d  Auf. 

Jena,  1913. 
Kellicott,  W.  E.     The  Social  Direction  of  Human  Evolution. 

New  York,  1911. 
Lock,  R.  H.     Variation,  Heredity  and  Evolution.     New  York, 

1911. 
Loeb,  J.     Comparative  Physiology  of  the  Brain  and  Comparative 

Psychology.     New  York,  1900. 

Loeb,  J.    The  Dynamics  of  Living  Matter.     New  York,  1906. 
Loeb,  J.    The  Mechanistic  Conception  of  Life.    Chicago,  1911. 
Loeb,  J.    Artificial  Parthenogenesis.    Chicago,  1913. 
Metchnikoff,  E.     The  Nature  of  Man.     Chicago,  1903. 
Morgan,  T.  H.    Heredity  and  Sex.    New  York,  1913. 
Morgan,  T.  H.    A  Critique  of  the  Theory  of  Evolution.    Prince- 
ton University  Press.     1916. 
Morgan,  T.  H.    The  Physical  Basis  of  Heredity.     Philadelphia, 

1919. 
Morgan,  Sturtevant,  Muller  and  Bridges.     The  Mechanism  of 

Mendelian  Heredity.     New  York,  1915. 
Mott,  F.  W.     Heredity  and  Eugenics  in  Relation  to  Insanity. 

London,  1912. 
Nageli,   C.     Mechanische-Physiologische   Theorie   der   Abstam- 

mungslehre.    Mimchen,  1884. 
Plate,  L.     Vererbungslehre.     Leipzig,  1913. 
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Eug.  Cong.    London,  1912. 
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48,  1906. 

Toyama,  K.  Maternal  Inheritance  and  Mendelism.  Jour.  Ge- 
netics 2,  1913. 

Weeks,  D.  F.  The  Inheritance  of  Epilepsy.  Problems  in  Eugen- 
ics, I. 

Whitman,  C.  O.  Animal  Behavior.  Biol.  Lectures.  Woods 
Hole,  1899. 

Whitney,  D.  D.  The  Effects  of  Alcohol  not  inherited  in  Hydatina 
senta.  Amer.  Nat.  46,  1912. 


352  Heredity  and  Environment 

Whitney,  D.  D.     The  Influence  of  Food  in  Controlling  Sex  in 

Hydatina  senta.    Jour.  Exp.  Zool.  17,  1914. 
Whitney;  D.  D.    Sex  Controlled  in  Rotifers  by  Food,    Abstracts 

of  Papers  Am.  Soc.  Zoologists,  Dec.  1915. 
Wieman,  H.  L.     The  Chromosomes  of  Human  Spermatocytes. 

Amer.  Jour.  Anat.  21,  1917. 
Wiesner,  J.     Die  Elementar-structur   und   das   Wachstum  der 

lebenden  Substanz.  Wien,  1892. 
Wilder,  H.  H.    Duplicate  Twins  and  Double  Monsters.    Amer. 

Jour.  Anat.  3,  1904. 
Wilson,  E.  B.    Some  Aspects  of  Cytology  in  Relation  to  the  Study 

of  Genetics.    Am.  Nat.,  1912. 
Wilson,  E.  B.     Studies  on  Chromosomes.     I- VIII.     Jour.  Exp. 

Zool.  2,  3,  6,  9,  13.    Jour.  Morph.  22. 
Winiwarter,  H.    Etudes  sur  spermatogenese  humain.    Archiv  d. 

Biologic,  27,  1912. 

Wolff,  C.  F.     Theoria  Generationis.     1759. 
Woltereck,  R.     Beitrag  zur  Analyse  der  Vererbung  erworbener 

Eigenschaften ;  Transmutation  und  Prainduktion  bei  Daphnia. 

Verh.  D.  Zool.  Ges.,  1911. 


GLOSSARY 

ACCESSORY  CHRO'-MO-SOME.  An  odd  chromosome  which  is  found  in  only 
half  of  the  spermatozoa  of  certain  animals;  see  "sex-chromosome." 

A-CHRO'-MA-TIN.  The  non-staining  substance  of  the  nucleus  as  contrasted 
with  the  chromatin. 

A-CHON'-DRO-PLA-SY.  A  condition  in  which  the  long  bones  cease  to  grow 
in  length  at  an  early  age  thus  producing  a  dwarf  with  large  body  and 
head  but  short  limbs. 

ACQUIRED  CHARACTER.  A  character,  the  differential  cause  of  which  is  en- 
vironmental. 

ALLELMORPH.     Contrasting  unit  characters,  or  their  corresponding  genes. 

ALTERNATIVE  INHERITANCE.  Galton's  term  for  a  doubtful  kind  of  inheri- 
tance in  which  all  characters  are  derived  from  one  parent.  In  present 
use,  Mendelian  inheritance. 

AM'-NI-ON.    One  of  the  embryonic  membranes  of  higher  vertebrates. 

AM-PHI-OX'-US.  One  of  the  lowest  and  simplest  animals  having  a  noto- 
chord  (backbone). 

AN-EN-CEPH'-A-LY.     The  condition  of  a  brainless  monster. 

ANIMAL  POLE.    That  pole  of  an  egg  at  which  the  polar  bodies  are  formed. 

AN'-LA-GE.    The  embryonic  basis  of  any  developed  part. 

A-OR'-TA.    The  great  artery  arising  from  the  heart. 

AR' -CHI-PLASM.     The  deeply  staining  plasm  surrounding  the  centrosome. 

AS'-CA-RIS.    A  genus  of  round  worms  which  are  intestinal  parasites. 

AS'-CA-RIS  meg-a-lo-ceph'-a-la.    A  parasite  in  the  intestine  of  the  horse. 

AS-CID'-I-AN.  A  "sea-squirt" ;  one  of  the  lowest  types  having  a  notochord, 
or  elementary  backbone. 

AS'-TER.    The  radiating  figure  surrounding  the  centrosome  in  a  cell. 

AS-SIM-I-LA'-TION.  Conversion  of  food  substances  by  an  organism  into 
its  own  living  substance. 

A-SYM'-ME-TRY.     The  condition  where  opposite  sides  are  unlike. 

AT'-A-VISM.  The  condition  in  which  an  individual  resembles  a  grand- 
parent, or  a  more  distant  ancestor,  more  than  one  of  the  parents. 

BI'-O-PHORES.    The  ultimate  units  of  life  (Weismann). 

BI'-O-TYPE.  A  group  of  individuals  all  of  which  have  the  same  Geno- 
type (Johannsen). 

BI'-VA-LENT  CHRO'-MO-SOMES.  A  pair  of  chromosomes,  one  maternal  the 
other  paternal,  temporarily  united. 

BLAS'-TO-COEL.    The  cavity  within  a  blastula. 

353 


354  Glossary 

BLAS'TO-DER'-MIC  VES'-I-CLE.  A  hollow  sphere,  formed  from  the  segmented 
egg  of  a  mammal,  which  becomes  attached  to  or  embedded  within  the 
wall  of  the  uterus. 

BLAS'-TO-PORE.    The  mouth  of  a  gastrula. 

BLAS'-TU-LA.  A  mass  of  cells,  usually  in  the  shape  of  a  hollow  sphere, 
formed  by  repeated  divisions  (cleavages)  of  an  egg. 

BLENDING  INHERITANCE,  Galton's  term  for  that  kind  of  inheritance  in 
which  the  characters  of  the  parents  seem  to  blend  in  the  offspring. 

BRACH-Y-DAC'-TY-LISM.  The  condition  of  having  abnormally  short  fingers 
or  toes. 

CELL.    The  fundamental  unit  of  structure  and  function  in  all  living  things. 

CEN'-TRO-SOME.    The  body  at  the  center  of  radiations  in  a  dividing  cell. 

CEPH'-A-LO-PODS.  A  class  of  mollusks  which  includes  the  squid,  cuttle- 
fish and  devil-fish. 

CER'-E-BRAL  GANG'-LI-ON.     The  brain  of  an  invertebrate  animal. 

CHARACTER.    Any  feature  or  property  of  an  organism. 

CHOR'-DA.  A  cellular  rod  in  vertebrate  embryos  which  forms  the  basis  of 
the  backbone. 

CHOR'-DATE.  A  member  of  the  highest  phylum  of  the  animal  kingdom, 
including  all  animals  having  a  chorda  or  backbone. 

CHO'-RI-ON.  A  tough  membrane  around  an  egg  secreted  by  surrounding 
cells. 

CHRO'-MA-TIN.     The  deeply  staining  substance  of  the  nucleus. 

CHRO'-MO-MERES.     The  linear  series  of  chromatin  bodies  in  a  chromosome. 

CHRO'-MO-SOMES.  Deeply  staining  bodies  found  in  the  nucleus  at  the 
time  of  indirect  division. 

CHROMOSOMAL  VESICLES.  The  swollen  chromosomes  of  inter-division 
stages. 

CIL'-I-A.  Minute  protoplasmic  threads  on  the  surface  of  a  cell  which  pro- 
duce movements  in  the  surrounding  medium  by  waving  back  and 
forth.- 

CLASS.    The  chief  sub-division  of  a  phylum. 

CLEAV'-AGE.    The  division  of  the  egg  cell  into  many  cells  after  fertilization. 

CLEP-SI'-NE.    A  genus  of  leeches. 

COE'-LOM.    The  body  cavity. 

CONTINUOUS  VARIATION.    A  series  of  minute  variations. 

CORRELATIVE  DIFFERENTIATION.  Differentiation  due  chiefly  to  the  interac- 
tion of  different  parts  of  an  organism. 

CRE-PID'-U-LA.    A  genus  of  marine  gastropods. 

"CRISS-CROSS"  INHERITANCE.  Morgan's  term  for  that  kind  of  inheritance 
in  which  maternal  characters  are  transmitted  to  sons  and  paternal 
ones  to  daughters. 


Glossary  355 

"CROSS-OVERS."  The  regrouping  of  linked  characters,  probably  caused 
by  interchange  of  genes  between  bivalent  chromosomes. 

CTEN'-O-PHORE.  A  jelly-sphere;  a  member  of  a  phylum  of  marine  animals 
standing  above  the  jelly-fishes. 

CY-CLO'-PI-A.  A  monstrosity  in  which  both  eyes  have  fused  into  a  single 
one. 

CY-TOL'-O-GY.    The  science  which  treats  of  cells. 

CY'-TO-PLASM.    The  protoplasm  of  a  cell  outside  of  the  nucleus. 

DAL' -TON-ISM.  That  form  of  color-blindness  in  which  one  is  unable  to  dis- 
tinguish red  and  green;  usually  limited  to  males. 

DAR'-WIN-ISM.  The  doctrine  that  evolution  takes  place  through  natural 
selection  or  the  survival  of  the  fittest. 

DETERMINANTS.     The  units  of  heredity   (Weismann). 

DETERMINER.  The  differential  cause  or  factor  in  a  germ  cell  which  deter- 
mines the  development  of  a  character. 

DEX'-TRAL  SNAIL.  The  usual  type  of  snail  in  which  the  shell  coils  from 
base  to  apex  in  a  clockwise  direction. 

DIFFERENTIATION.  The  process  of  producing  specific  parts  or  substances 
from  a  general  part  or  substance. 

DI-HY'-BRID.     The  offspring  of  parents  differing  in  two  characters. 

DI-O-NAE'-A.    An  insect-catching  plant,  the  "Venus  Fly-trap." 

DIP'-LOID.  The  full  number  of  chromosomes  found  in  the  fertilized  egg 
and  in  all  cells  derived  from  this,  except  the  mature  germ  cells. 

DOMINANT  CHARACTER.  A  character  inherited  from  one  parent  which  de- 
velops to  the  exclusion  of  a  contrasting  character  of  the  other  parent. 

DROS-OPH'-I-LA.    A  genus  of  fruit-flies. 

DU'-PLEX  FACTORS  or  CHARACTER.  A  condition  where  the  determiners  for 
a  character  are  derived  from  both  parents. 

E-CHI'-NO-DERMS.  A  phylum  of  marine  animals  which  includes  star-fishes 
and  sea-urchins. 

E-COL'-O-GY.  The  science  which  deals  with  the  relations  of  organisms  to 
one  another  and  to  environment. 

EC'-TO-DERM.  The  outer  layer  of  cells  of  an  embryo  which  gives  rise  to 
epidermis,  sense  organs  and  nervous  system. 

EM-BRY-OG'-E-NY.  Early  development  of  an  egg  leading  to  the  formation 
of  an  embryo. 

EN'-DO-DERM.  The  inner  layer  of  cells  of  an  embryo,  which  gives  rise  to 
the  digestive  cells  of  the  alimentary  system. 

EN-DO-GEN'E-SIS  (=  development  from  within).  The  theory  that  the 
differential  causes  of  development  are  within  the  germ  cells,  which 
are  therefore  complex. 


356  Glossary 

EP-I-GEN'E-SIS  (—  development  from  without).  The  doctrine  that  the 
germ  is  simple  and  homogeneous  and  that  the  differential  causes  of 
development  are  in  the  environment. 

EQUATION-DIVISION.  An  ordinary  nuclear  division  in  which  each  chro- 
mosome divides  equally. 

EU-GEN'-ICS.    The  system  of  improving  races  by  good  breeding. 

EU-THEN'-ICS.    The  system  of  improving  individuals  by  good  environment. 

EX-O-GAS'-TRU-LA.    A  gastrula  with  the  endoderm  turned  out  instead  of  in. 

FACTOR.    A  specific  germinal  cause  of  a  developed  character. 

FERTILIZATION.    The  union  of  male  and  female  sex  cells. 

FLA-GEL'-LUM.  A  vibratile  thread  of  protoplasm  which  serves  as  an  or- 
gan of  locomotion. 

FLUCTUATIONS.     Variations   which   are   not   inherited. 

FOL'-LJ-CLE  CELLS.    Nutritive  cells  surrounding  an  ovarian  egg. 

FRATERNAL  TWINS.  Twins  produced  from  different  eggs  and  showing 
different  hereditary  characters. 

FUNCTIONAL  ACTIVITY.    Use. 

GAM'-ETE.    The  mature  male  or  female  sex  cell. 

GANG'-LI-ON.    A  group  of  nerve  cells. 

GAS'-TRO-COEL.    The  digestive  cavity  of  the  gastrula. 

GAS'-TRU-LA.  A  stage  in  development  following  the  blastula,  in  which  the 
embryo  consists  of  an  outer  (ectoderm)  and  an  inner  (endoderm) 
layer  of  cells. 

GENES.  Factors,  units,  elements  of  germ  cells  which  condition  the  char- 
acters of  developed  organisms  (Johannsen). 

GE-NET'-ICS.  The  science  which  deals  with  the  origin  of  individuals  and 
particularly  with  heredity. 

GE'-NO-TYPE.  The  germinal  type  with  all  its  hereditary  peculiarities.  "The 
fundamental  hereditary  constitution  of  an  organism"  (Johannsen). 

GERM-PLASM.    The  material  basis  of  inheritance. 

GERM-TRACK.    The  cell-lineage  of  the  germ  cells  in  a  developing  animal. 

GERMINAL  UNITS.  Hypothetical  parts  of  germ  cells  which  are  supposed 
to  have  certain  specific  functions  in  development. 

HAE-MO-PHIL'-I-A.  An  abnormal  condition  in  which  the  blood  clots 
slowly. 

HAP'-LOID.    The  reduced  number  of  chromosomes  in  the  gametes. 

HEREDITY.  The  appearance  in  offspring  of  characters  whose  differential 
causes  are  in  the  germ  cells. 

HERITAGE.  The  sum  of  those  characters  which  are  inherited  by  an 
individual. 

HET-ER-O'-SIS.    Increased  vigor  due  to  hybridity 


Glossary  357 

HET-ER-O-ZY-GO'-SIS.    Hybridization ;  cross-breeding. 

HET-ER-O-ZY'-GOTES.  Hybrids  resulting  from  the  union  of  gametes  which 
are  hereditarily  dissimilar. 

HO-MO-ZY'-GOTES.  Pure-breds  resulting  from  the  union  of  gametes  which 
are  hereditarily  similar. 

HY'-BRID.    The  offspring  of  parents  which  differ  in  one  or  more  characters. 

IDENTICAL  TWINS.  Twins  which  have  come  from  a  single  egg  and  which 
show  identical  hereditary  characters. 

ID'-I-O-PLASM.     The  germ-plasm  or  inheritance  material. 

INDUCTION.  A  modification  of  the  first  filial  generation  caused  by  the 
action  of  environment  on  the  germ  cells  of  the  parental  generation. 
(Woltereck.) 

INHERITED  CHARACTER.  A  character  the  differential  cause  of  which  is  in 
the  germ. 

INSTINCTS.     Complex  reflexes  involving  nerve  centers. 

INVERSE  SYMMETRY.  Having  the  right  half  of  one  asymmetrical  individual 
equivalent  to  the  left  of  another;  mirrored  symmetry. 

IRRITABILITY.    Capacity  of  receiving  and  responding  to  stimuli. 

KAR-Y-O-KI-NE'-SIS.     See  Mitosis. 

LA-MARCK'-ISM.  The  doctrine  that  evolution  takes  place  through  the  in- 
heritance of  acquired  characters. 

LETHAL  FACTORS.  Factors  which  cause  the  early  death  of  gametes  or  zy- 
gotes. 

LINKAGE.  Inheritance  of  characters  in  groups,  probably  due  to  the  link- 
age of  genes  in  a  chromosome. 

LOCALIZATION.  The  gathering  together  of  particular  substances  in  definite 
parts  of  an  egg  or  embryo. 

LOL'-I-GO.    The  squid,  a  genus  of  cephalopod  mollusks. 

MAR-SU'-PI-ALS.  A  primitive  group  of  mammals,  including  opossums  and 
kangaroos,  which  carry  the  young  in  a  pouch. 

MAT-U-RA'-TION.  The  final  stages  in  the  formation  of  sex  cells,  charac- 
terized by  two  peculiar  cell  divisions. 

ME-RIS'-TJC  VARIATION.    Variation  in  the  number  of  parts. 

MES'-EN-CHYME.    Loosely  scattered  cells  of  the  mesoderm. 

MES'-O-DERM.  A  layer  or  group  of  embryonic  cells  lying  between  ectoderm 
and  endoderm. 

ME-TAB'-O-LISM.  Transformation  of  matter  and  energy  within  a  living 
thing. 

MI'-CRO-PYLE.  The  minute  opening  in  an  egg  membrane  through  which 
the  spermatozoon  enters. 

MI-TO'-SIS.    Indirect  nuclear  division  in  which  the  nucleus  is  transformed 


358  Glossary 

into  a  spindle  and  chromosomes;  the  latter  split  and  the  halves  move 

to  the  poles  of  the  spindle  where  they  form  the  daughter  nuclei. 
MODIFYING  FACTORS.    Factors  whose  principal  influence  is  seen  in  modifying 

other  factors  or  the  characters  to  which  they  give  rise. 
MON-O-HY'-BRID.     The  offspring  of  parents  differing  in  one  character. 
MON'-O-TREMES.     The  lowest  group  of  mammals,  including  the  duck-bill 

and  the  spiny  anteater. 

MOR-PHOI/-O-GY.    The  science  which  deals  with  structure  and  form. 
MUS'-CA.    A  genus  of  flies  including  the  house-fly. 
MU'-TANT.    A  sudden  variation  or  sport  which  breeds  true. 
MU-TA'-TIONS.     Inherited    variations    which   are   more    or    less    striking; 

"sudden  variations,"  "sports." 

NEC-TU'-RUS.    A  large  salamander;  the  mud-puppy. 
NEM'-A-TODE.    A  round-worm  or  thread-worm. 
NE'-RE-IS.    A  marine  annelid,  or  ringed  worm. 
NEURAL  GROOVE.    The  groove  on  the  dorsal  surface  of  the  embryo  of  a 

vertebrate  which  develops  into  the  brain  and  spinal  cord. 
NEURAL  TUBE.    A  tube  formed  from  the  neural  groove  and  giving  rise  to 

brain  and  spinal  cord. 

NO'-TO-CHORD.    The  cellular  rod  which  forms  the  basis  of  the  backbone. 
NU'-CLE-US.     The  central  organ  of  a  cell,  composed  of  chromatin  and 

achromatin. 
NULLIPLEX  FACTORS  or  CHARACTER.    A  condition  in  which  a  character  is 

absent  because  its  determiner  is  found  in  neither  parent. 
ON-TOG'-E-NY.    Development  of  an  individual. 
O'-O-CYTE.     The    ovarian    egg   before    maturation    (formation   of    polar 

bodies). 

O-O-GEN'-E-SIS.    The  development  of  an  ovum  from  a  primitive  sex-cell. 
O-O-GO'-NI-A.    The  earliest  generations  of  cells  which  produce  ova;  pri- 
mordial egg  cells. 

O'-O-SPERM.    The  fertilized  egg  after  union  of  egg  and  sperm. 
ORDER.    The  chief  sub-division  of  a  class. 
ORGANIZATION.    Differentiation  and  integration,  i.e.,  different  parts  united 

into  one  whole. 

OR-GAN-OG'-E-NY.    The  formation  of  various  organs  of  the  body. 
OR-THO-GEN'-E-SIS.    The  doctrine  that  the  course  of  evolution  is  definitely 

directed  by  intrinsic  causes. 
O-VI-PAR'-I-TY.     Young  brought  forth  as  eggs,  i.e.,  in  an  early  stage  of 

development. 
O'-VULES.    The  female  sex  cells  of  flowering  plants  with  the  immediately 

surrounding  parts. 


Glossary  359 

O'-VUM.    The  female  sex  cell. 

OX-Y-CHRO'-MA-TIN.  That  portion  of  the  chromatin  which  does  not  form 
chromosomes. 

PAN-GEN'-E-SIS.  The  hypothesis  proposed  by  Darwin  that  every  cell  of  the 
body  gives  off  minute  germs,  "gemmules,"  which  then  collect  in  the 
sex  cells. 

PAR-A-ME'-CI-UM.    A  ciliated  protozoan. 

PAR-THE-NO-GEN'-E-SIS.  Development  of  an  egg  without  previous  fertili- 
zation. 

PARTICULATE  INHERITANCE.  Galton's  term  for  that  kind  of  inheritance  in 
which  certain  characters  are  derived  from  one  parent  and  others  from 
the  other  parent,  i.e.,  Mendelian  Inheritance. 

PA-THOL'O-GY.    The  science  which  deals  with  disease. 

PHE'-NO-TYPE.  The  developed  type  in  which  some  of  the  hereditary  pos- 
sibilities are  realized  while  others  remain  undeveloped.  "Developed, 
measurable  realities"  (Johannsen). 

PHY-LOG'-E-NY.    Evolution  of  a  race  or  species. 

PHYL-LOX'-E-RA.    A  genus  of  plant  lice. 

PHY'-LUM.    One  of  the  chief  sub-divisions  of  the  animal  kingdom. 

PHYS-I-OL'-O-GY.    The  science  which  deals  with  function. 

PLAS'-TO-SOMES.  Threads  or  granules  in  the  cytoplasm  which  are  colored 
by  certain  dyes. 

POLAR  BODIES.  Two  minute  cells  which  are  separated  from  the  egg  in  its 
two  maturation  divisions. 

PO-LAR'-I-TY.  The  condition  where  two  poles  of  a  body  differ;  in  eggs  the 
two  poles  are  the  animal  (formative)  and  the  vegetative  (nutritive). 

POL'-LEN.    The  male  sex  cells  of  flowering  plants. 

POL-Y-DAC'-TYL-ISM.  The  condition  of  having  more  than  the  normal  num- 
ber of  digits  on  hands  or  feet. 

POL-Y-HY'-BRID.  The  offspring  of  parents  differing  in  more  than  three 
characters. 

PRE-FOR-MA'-TION.  The  doctrine  that  the  fully  formed  organism  exists 
in  the  germ,  and  that  development  is  merely  its  unfolding. 

PRE-IN-DUC'-TION.  A  modification  of  the  second  filial  generation  caused 
by  the  action  of  environment  on  the  germ  cells  of  the  parental  genera- 
tion. (Woltereck.) 

PRE-IN-HER'-IT-ANCE.  The  transmission  of  characters  developed  in  a  pre- 
vious generation. 

PRE-PO'-TENCY.  The  preponderance  of  one  parent  over  the  other  in  the 
transmission  of  hereditary  characters. 

PRI'-MATES.  The  highest  order  of  mammals  including  monkeys,  apes,  and 
man. 


360  Glossary 

PRIMITIVE  SEX  CELLS.  The  earliest  recognizable  progenitors  of  the  sex 
cells  in  development. 

PRO'-TE-IN.  Complex  organic  substances  containing  nitrogen,  e.g.  white 
of  egg. 

PRO-TE'-NOR.    A  genus  of  the  true  bugs. 

PRO'-TO-PLASM.    The  living  material  of  an  organism. 

PRO-TO-ZO'-A.    The  simplest  animals,  usually  consisting  of  a  single  cell. 

PY-LO'-RUS.    The  narrow  opening  between  stomach  and  intestine. 

RECESSIVE  CHARACTER.  An  inherited  character  which  remains  undeveloped 
when  mated  with  a  dominant  character. 

REDUCTION-DIVISION.  That  maturation  division  in  which  the  number  of 
chromosomes  is  halved. 

REFLEXES.     Relatively  simple,  automatic  responses. 

RESPONSE.    Any  activity  of  an  organism  called  forth  by  a  stimulus. 

REVERSIONS.    The  sudden  reappearance  of  long-lost  racial  characters. 

SEGREGATION.  The  separation  of  contrasting  parental  characters  in  the 
offspring  of  hybrids,  or  of  contrasting  genes  (allelomorphs)  in  the 
formation  of  gametes. 

SELF  DIFFERENTIATION.     Differentiation  due  chiefly  to  intrinsic  causes. 

SENSITIVITY.    Capacity  of  receiving  and  responding  to  stimuli. 

SEX  CHRO'-MO-SOME.  The  "odd"  or  accessory  chromosome  which  is  sup- 
posed to  determine  sex. 

SEX-LIMITED.    Any  character  which  is  found  in  one  sex  only. 

SEX-LINKED.  Any  character,  the  determiner  of  which  is  associated  with 
the  determiner  of  sex. 

SIMPLEX  FACTORS  or  CHARACTER.  A  condition  where  the  determiner  for 
a  character  is  derived  from  one  parent  only. 

SIN'-IS-TRAL  SNAIL.  A  type  of  snail  in  which  the  shell  coils  from  base  to 
apex  in  an  anti-clockwise  direction. 

SO'-MA.    The  body  as  contrasted  with  the  germ  cells. 

SO-MAT'-IC.  Pertaining  to  the  body,  as  contrasted  with  "germinal"  pertain- 
ing to  the  germ  cells. 

SO'-MA-TO-PLASM.    The  body-plasm  as  contrasted  with  the  germ-plasm. 

SO'-MITE.    A  segment  of  the  body  of  a  segmented  animal. 

SPER-MA'-TO-CYTES.    The  mother  and  grandmother  cells  of  spermatozoa. 

SPER-MA-TO-GEN'-E-SIS.  The  development  of  a  spermatozoon  from  a  prim- 
itive sex  cell. 

SPER-MA-TO-GO'-NI-A.    Primordial  sperm  cells. 

SPER-MA-TO-ZO'-ON.    The  mature  male  sex  cell. 

SPINDLE.    The  nuclear  division  figure. 

SPI'-REME.  A  coiled  thread  of  chromatin  which  appears  in  the  nucleus  at 
the  beginning  of  mitosis. 


Glossary  361 

SPI-RIL'-LA.    A  spiral  type  of  bacteria. 

STEN'-TOR.    A  ciliated  protozoan. 

STER-E-O-I'-SO-MERES.  Molecules  having  the  same  composition  but  differ- 
ent properties  dependent  upon  varying  spatial  relations  of  their  con- 
stituent atoms. 

STIM'-U-LUS.    Anything  acting  on  an  organism  which  calls  forth  a  response. 

STY-E'-LA.    A  genus  of  Ascidians. 

SYM'-ME-TRY.     The  condition  where   opposite   sides   or  poles  are  alike ; 

bilateral,  having  equivalent  right  and  left  sides. 
SYN-AP'-SIS.     The   conjugation   of   maternal   and   paternal   chromosomes 

preceding  the  maturation  divisions. 

SYN-DAC'-TYL-ISM.    The  condition  of  having  webbed  fingers  or  toes. 
TE-NEB'-RI-O.     A  genus  of  beetles,  the  larva  of   which  is  the  common 

meal  worm. 
TER-A-TOL'-O-GY.    The  science  which  deals  with  monstrous  or  abnormal 

forms. 

TET'-RADS.    Bivalent  chromosomes  which  appear  4-parted  in  the  matura- 
tion divisions. 
TO-TIP'-O-TENCE.     The  capacity  of  a  cleavage  cell  to  give  rise  to  a  whole 

animal. 
TOX'-IN.     A   poisonous   substance   particularly   such   as   is   produced  by 

bacteria. 

TRI-HY'-BRID.    The  offspring  of  parents  differing  in  three  characters. 
TROPH'-O-BLAST.    The  outer  layer  of  the  blastodermic  vesicle  of  a  mammal. 
TRO'-PISMS.    Automatic  movements  of  organisms  toward  or  away  from  a 

source  of  stimulus. 
UNIT  CHARACTER.    A  character  which  is  inherited  as  a  whole  and  cannot 

be  sub-divided. 

VEGETATIVE  POLE.    The  pole  of  an  egg  opposite  the  polar  bodies. 
VIL'-LI.    Processes  which  grow  out  from  the  embryonic  membranes  of  a 

mammal  and  connect  it  to  the  walls  of  the  uterus. 
VI-TEL'-LINE  MEMBRANE.    A  delicate  membrane  around  an  egg  secreted  by 

the  egg  itself. 
VIV-I-PAR'-I-TY.    Young  brought  forth  "alive,"  i.e.,  in  an  advanced  stage 

of  development. 
ZY'-GOTE.    The  product  of  the  union  of  male  and  female  sex  cells. 


INDEX 


Proper  names  and  titles  of  sections  are  in  small  capitals;  page  refer- 
ences to  illustrations  in  italic  numerals. 


Aborigines  of  Australia,  290 

New  Zealand,  290,  300 

North  America,  290 

Pacific  Islands,  290 

South  America,  290 

Tasmania,  290 

West  Indies,  290 
"Accessory"  chromosome,  180 
Achondroplasy,  69,  117 
Achromatin,  136 

differential  distribution,  146,  209 

in  cell  body,  205 
ACKERT,  Effects  of  Selection  on  Para- 

mecium,  269 

ACQUIRED   CHARACTERS,   INHERITANCE 
OF,  237-249 

Darwin  on,  237 

Lamarck  on,  237 

Weismann  on,  238 

definition  of,  238 

general  objections  to,  239 

specific  objections  to,  240-247 
Adaptive  responses,  46,  47 
Adrenal  gland,  fed  to  tadpoles,  230 

influence  on  development,  235 
African  negro,  no,  293 

in  Jamaica,  300 

in  U.  S.,  300 
Age  of  human  race,  287 
Albinism,  115,  119 
Alcoholism,  71,  118,  257 

cause  of  sterility,  stillbirths,  mal- 
formation, dwarfs,  218,  220,  246 

induction  effect  on  rotifers,  246 

influence  on  germ  cells,  218 
Alkaptonuria,  118 
Allelmorphs,  84.    Multiple,  08. 
Alpine  plants,  non-inheritance  of  ac- 
quired  characters,  249 
"Alternative"  inheritance,  73 
Amalgamation  of  races,  300 


American    men    of    science,    families 
of,  311 

Amphiaster,  17 

AMPHIOXUS,    cleavage    and    differen- 
tiation, 142 

cleavage  and  gastrulation,  22 
isolated  cleavage  cells,  222 

Ancestors,  number  of,  77,  78 

Ancestry,  pride  of,  308 

ANCYRACANTHUS,  oogenesis,/55 
spermatogenesis,  154 

Andalusian  fowl,  Blue,  106 

Anencphaly,  228 

Animal  pole  of  egg,  n,  195 

Annelid  type  of  egg,  202 

Ants  and  bees,  231,  314,  337 

Artificial  limitation  of   families,  311, 
313 

Artificial    parthenogenesis,,    132,    172, 
221 

ARTIFICIAL  SELECTION,  266 

chief  factor  in  production  of  do- 
mestic races,  266,  292 
creates  nothing,  266,  272 
isolates  pure  lines,  266,  269 
lacking,  294 

ASCARIS,  fertilization  of,  137 
"germ  track,"  145,  749 
sex  differentiation  in,  161 

Ascidian  egg,  142,  143,  146,  147,  202, 
224,  225,  226 

Aster,  17 

ASTERIAS,  fertilization  of,  12 

Astral  radiations,  17 

Atavism,  73,  82 

ATHENS,  293 

Athletes,  prize,  209 

Athletics,  educational  value  of,  334 

ATTICA,  illustrious  men  of  293,  294 

Automaton,  325 

Autosomes,  170 


363 


3^4 


Index 


Backbone,  development  of,  23 
Bacteria,  reactions  to  light,  38 

to  salt,  39 
BAILEY,  number  of  cultivated  plants, 

259 

Baldness,  inherited,  68 
BALZAC,  heredity  a  maze,  119 
Barbarism,  256,  287,  295 
BARDEEN,  X-rays  on  spermatozoa,  218 
BATESON,  Blue  Andalusian,  106 

brachydactyl  hand,  115 

"homozygote.  and    heterozygote," 
84 

on  evolution,  282,  283 
BATESON  and  PUNNETT,  on  sweet  pea 

hybrids,  102,  104,  185 

linkage,  185 

BAUR,   factors  for  flowers  of  Antir- 
rhinum, 103 

on  leaf  colors,  114 
induction  effects  of  poor  soil,  246 
Beans,  pure  lines,  66,  267 
Bees,  and  Ants,  168,  315,  339 

influence    of    food    on    develop- 
ment, 231 

workers,  queens,  drones,  231 
BEETHOVEN,  334 
Behavior,  dogs,  cats,  monkeys,  48 

fish  and  frog,  330 

lower  organisms,  37-47 

modifiability  of,  50 

Paramecium,  46,  47,  48 

plasticity  of,  51 

rigidity  of,   51 

test  of  psychical  processes,  36 

worms,   star-fish,   Crustacea,   ver- 
tebrates, 47-51 

Biglow  Papers,  on  Lamarckism,  237 
"Biophores"  of  Weismann,  129 
Birth  Control,  313 

Bithrate     and     deathrate,     normally 
equal,  309 

both  decreasing.  310 

decreasing  most  in  best  families, 

3ii 

in  Massachusetts,  311 

Bivalent  chromosomes,  153 

Blastula,  22,  24,  25 

"BLENDING"  INHERITANCE,  73,  108-112 

in  length  of  ears  in  rabbits,  in 

in  length  of  skull  in  rabbits,  112 

in  skin  color  of  mulatto,  109,  in, 

114 


"Blood  lines,"  268 
Body  and  mind,  35 

parallel  development,  55 
BOULE,  Chapelle  aux  Saints  skull,  288 
BOVERI,  chromosomes  differ  in  value, 
174 

cleavage  of  sea  urchin  egg,  14 

dispermic  eggs,  221 
Brachydactylism,  69,  7/5,  117 
Brain,  development,  23 

size  and  weight,  334 
Breeder,  methods  of,  273,  274 
BRIDGES,  intersexes  in  Drosophila,  169 

non-disjunction     in     Drosophila, 

183,  184 

/?ROMAN,  death  of  families,  312 
BROOKS,  75 
Buddhistic    belief    in    transmigration, 

33 

BURBANK,  new  combinations  of  char- 
acters, 273 
BUTLER,  BISHOP,  321 

Cards,  comparison  with  chromosomes, 

176 

Canary  birds,  fed  on  red  pepper,  230 
CANNON,  relations  of  adrenin  to  pain, 

hunger,  fear  and  rage,  236 
physical    substitute    for    warfare, 

304 
Capacity,  greater  than  realization,  334, 

.  335 

Captivity,  cause  of  infertility,  313 
CASTLE,    factors    for   coat   colors    of 

rabbits,  101 
recombinations      of      characters, 

270,  271 

size  in  rabbits,  112 
value  of  selections,  268,  2/2 
CASTLE    and    PHILLIPS,    transplanted 

ovaries  of  guinea-pigs,  243,  244 
Cataract,  hereditary,  67,  118 
CATTELL,  birthrate  of  college  gradu- 
ates, 309 

families  of  scientists,  311 
Cause  and  effect,  universality  of,  320, 

321 
Causes,  natural  vs.  final,  182 

elibacy,  296,  309 
Cell  characters  inherited,  67 
CELL  DIVISION,  16,  18,  19,  20,  125,  135, 
139,  136,  145 


Index 


365 


differential,  206 

non-differential,  146,  206 

significance  of  144,  148 
Cell-Lineage,  diagram  of,  126 
CELLULAR  BASIS  OF  HEREDITY,  123 
Centrifuged  eggs,  228 
Centrosomes,  8,  n,  17 

equal  division  of  144,  206 
Chances,  definition  of,  329 

infinity  of  in  development,  306 
CHARACTERS,     developed    not    trans- 
mitted, 124 

individual,  65 

inheritance  of  acquired,  237-249 

inherited,  definition  of,  64,  240 

latent,  73 

NEW    COMBINATIONS,    72 

NEW,  or  MUTATIONS,  74 

new  in  evolution,  276-286 

not  independent,  64 

patent,  73 

racial,  65 

CHILD,  on  chromosomes,  182 
Choice,  conscious,  52 

of  alternatives,  331 
Chorea,  120 
Chromatin,  8,  127,  136 

granules,  16,  138 

as  germplasm,  129 
Chronomeres,  equal  division  of,   144, 

206 
Chromosomes,  16 

abnormal    distribution,    182,    221, 
281,  282 

accessory,  158 

bivalent,  153 

compared    with    digits,    151,    152, 
156,  161 

compared  with  cards,  176 

conjugation  of,  151 

daughter,  138 

diploid,  156 

distribution,  138 

division,  17,  144,  146 

of  egg  and  sperm,  137 

haploid,  156 

identity,  138 

map  of,  194 

maternal  and  paternal,   155,  137, 

141,  175 

non-disjunction  of,  183,  184 
number  of,  16,  138 
number  in  man,  162,  163,  164 


permutations  of,  173,  175 

"odd,"  158 

reduction  of,  153,  157 

seat  of  factors,  130,  179 

shuffle  and  deal  of,  176 

tetrads,  153 

X  and  Y,  160,  161 

Chromosomal  vesicles,  17,  20,  136,  138 
CHROMOSOMAL  INHERITANCE  THEORY, 

179,  195 

applied  to  embryonic  differentia- 
tion, 207 

LOCALIZATION,  178,  194 
Civilization,     means     good     environ- 
ment, 215 
vs.  heredity,  255 
will  it  endure,  287,  341 
Classes,  hereditary,  288 

exclusive,  299 
CLEAVAGE,  of  egg,  14,  21 

AND  DIFFERENTIATION,   136 

differential,   145,  207 

non-differential,  207 

significance  of,  144,  148 
Cleavage  cells,  differentiation  of,  21, 
140,  148 

isolated     development     of,     222, 

224-226 

CLEPSINE,  behavior  of,  51 
Climatic  effects  not  inherited,  245 
Coeducation,  308 

Cold,    induction    effect    on    Daphnia, 
246 

induction  effect  on  mice,  246 
Coloboma,  69,  118 
Color,  of  skin,  hair,  eyes,  109,  ///, 

114,  117 

Color-blindness,  119,  188,  189 
Conjugation  of  homologous  chromo- 
somes, 151 
CONSCIOUSNESS,  53,  54 

continuity  of,  54 

loss  of,  54 

subconscious,  53 
Contributors,  77-79 
CONTROL  OF,  alternatives,  327 

development  and  evolution,  4 

HUMAN  EVOLUTION,  29! 

meaning  of,  328 
nature,  4 

phenomena,  259,  328 
self,  328,  335 
CORRELATIONS  OF  GERM  AND  SOMA,  178 


366 


Index 


"Correlative  differentiation,"  235 
CORRENS,     rediscovery    of    "Mendel's 
Law,"  82 

on  Mirabilis,  87,  98,  106 

leaf  colors,  114 

COTTON,     insanity     not     directly     in- 
herited,   71. 
Creationism,  33 
Creative  Synthesis,  31,  59,  205 
CREPIDULA,  maturation  and   fertiliza- 
tion, 135 

individuality  of  germ  nuclei,  141 

exogastrula  of,  229 
Cretin,  233 
Criminality,  255,  320 
"Cross  Overs,"  193,  194 
CTENOPHONE,  egg,  203 
CULTIVATED  PLANTS,  259 

number  of  species,  259 
Culture,  grades  of,  287 
CUVTER,  334 
Cyclopia,  230 
Cytoplasm,  8 

differentiates,  nuclei   do  not,  145 

distribution  of,  140 

is  somatoplasm,   128 
CYTOPLASM ic  CORRELATIONS,  196 

differentiations,  145,  148 

inheritance,  196 

localization,   142 

movements,  140 

Daltonism,  sex-linked,  188,  189 
DAPHNIA,  effect  of  cold  on,  246 
DARWIN,    hypothesis    of    pangenesis, 
124 

on  domestic  pigeons,  82 

inheritance    of    acquired    charac- 
ters, 237 

linked  characters,  185 

prepotency,  82 

reversion,  82 

"sports,"  74 

zeal,  334 

organism  of  microcosm,  210 

theory  of  Natural   Selection,  266 
DAVENPORT,    degrees    of    relationship, 
78 

extra  toe  in  fowls,  107 

inheritance  of  skin  color,  109-111 

Mendelian    inheritance    in    man, 


transplanted  ovaries,  242 
"weakness  with  strength,"  307 
white  X  black  leghorns,  107 

DAVENPORT  AND  WEEKS,  epilepsy  in^ 
herited,  71 

DAVIS,  on  Oenothera  hybrids,  2/7 

Deaf-mutism,  69,  118 

Death,  of  families,  311 

Deathrate,  declining,  310 

DECANDOLLE,  number  of  food  plants 

259 

Declaration  of  Independence,  214 
Decline  of   families  and  nations,  341 
Defectives,  growing  burden  of,  296 

alarming  increase  of,  302 
Defects,  educational  influence  of,  251 
Democracy  and  human  equality,  214 
DESCARTES,  214 

"Determiants"  of  Weismann,  129 
Determiners,  99,  130 

combinations  of,  100 

differential  causes,  99 
DETERMINISM, 

and  RESPONSIBILITY,  327 

definition  of,  327 

not  FATALISM,  327 

not  predeterminism,  328 

of  ENVIRONMENT,  324 

of  HEREDITY,  321 

of  personality,  327 

scientific,  328 

Development,   a  series  of   responses 
231 

alternatives  in,  329 
I          definition  of,  132 

is    transformation    not   new    for- 
mation, 177 

mosaic,  223 

not  reversible,  328 

of  function,  29 

of  personality,  55,  319,  328 

potentialities  of,  325 

physiology  of  216 

various  aspects  of,  55 

viviparous,  26 
DEVELOPMENT  OF  BODY,  6-32 

OF  MIND,  32-56 

tabular  summary,  56 
DEVELOPMENTAL  RESPONSES,  218 

movements,  231 

AFTER  FERTILIZATION,  221 
BEFORE  FERTILIZATION,  2l8 
DURING  FERTILIZATION,  221 


Index 


367 


DE  VRIES,  action  of  selection,  267,  272 

fluctuations,  74,  75 

induction  effects  of  poor  soil,  246 

intra-cellular  pangenesis,  206 

"mutation  theory,"  74,  274 

mutations,  74,  274,  279 

Oenothera  mutants,  274,  278,  279- 
282 

on  nuclear  control  of  differentia- 
tion, 205 

pangenes,  129 

rediscovery   of    "Mendel's    Law," 

82 

Diabetes,  118 
DIDEROT,  214 
Digits  compared  to  chromosomes, 

151,  153,.  156,  161 

Differential    division    of    Cytoplasm, 
145,  148 

of  cells,  145,  206 
Differentiation,  31 

"correlative,"  235 

definition  of,  132 

due   to   interaction   of   cell   parts, 
178,  204 

measure  of,  207 

nuclear  control  of,  204 

"self,"  237 
Dihybrid,  92,  93 
Dimples,  inheritance  of,  66 
DIONAEA,  reactions  of,  44 
Diploid  number  of  chromosomes,  156 
Disease,  inheritance  of,  69 

slight  resistance  to,  70 
Dislocation  of  organs  in  centrifuged 

eggs,  227,  228 
Dispermic  eggs,  183,  221 
Divines,  poor  health,  252 
Division  period,  148 
Dogs,  different  races,  261 

psychological     characters     inher- 
ited, 70 
DOMESTIC  ANIMALS,  259 

degree  of  change,  259 

how  produced,  264,  273 

number  of  species,  259 

progenitors,  259 

regressive  mutants,  282 
DOMINANCE,  MODIFICATIONS  OF,  106 

Blue  Andalusian,  106 

echinoderm  hybrids,   107 

extra  toe  in  fowls,  107 

incomplete,  106 


in  red  X  white  Mirabilis,  87 
nature  of,  107 
not  fundamental,  107 
plain  X  banded  snails,  106 
red  X  white  cattle,  106 
reversible,  107 
sex-limited  characters,  171 
sex-linked  characters,  185 
white  X  black  leghorns,  107 

Dominant  characters,  84 
"extracted,"  86 
ratio  to  recessive,  84,  88 
races,  290,  291 

Double   monsters,  223,  224,  229 

DRIESCH,  236 

DROSOPHILA,    chromosomes    of,    1 

.       W,  194 
"cross  overs,"  192,  193 
modifying  factors,  103 
lethal  factors,  105,  190 
mutants,  284,  285,  286 
rapid  breeding  of,  114 
sex-linked  characters,  186,  187 

DRYDEN,  71 

Duplex  Factors,  98 

Duty,  319,  322 

of  science,  337 

Dwarfs,  true,  117 

caused  by  alcohol,  220 

Dynamic  equilibrium,  8 

Ear,  development,  23 
EAST,  heterosis,  273  274,  275 
ECHINODERM  type  of  egg,  202 
ECHINUS,  first  cleavage,  14 
Ectoderm,  23 
Education  and  heredity,  307 

capacity  for,  250 

definition,  250,  343 

good  and  bad,  251,  252 

habit  formation,  326 

limiting  activities,  251 

more  potent  in  man,  249 

needs  of,  336 

possible  improvements,  343 
Egg  and  sperm,  10 

hereditary  inequality  of,  19^ 
Egg  nucleus,  21 

Egg  organization,  types  of,  202 
ELSBERG,  plastidules,  129 
Emboitement,  57 
EMBRYOGENY,  23 
Fmbryology,  experimental,  216 


368 


Index 


Embryonic    differentiation,    processes 

in,  203,  208 
Embryos,  double,  222,  223,  229 

dwarf,  224 

half  and  three  quarter,  224-226 
Endoderm,  23 

ENDOGENESIS  AND  EPIGENESIS,  58 
"Energies  of  Men,"  333 
ENGLEMANN,  38 
English  sparrow  in  U.  S.,  309 
Engrammes,  246 

Environment,  acting  at  sensitive  per- 
iod, 266 

definition,  216 

direct  action  on  germ  cells,  246- 
248 

and  education,  252  « 

good  and  bad,  251-254,  335 

influence  in  producing  new  races, 
264,  266 

influence  on  ontogeny,  214,  216 

influence  o  i  phylogeny,  214,  280 

larger  influence  on  man,  249 

non-specific,  172 

possible  improvements,  343 

social  institutions,  214 
Epidermolysis,  117 
Epigenesis,  57,  58,  177,  204,  324 
Epilepsy,  71,  118 
Equality  of  Man,  214,  322 
Equatorial  plate,  16,  17 
Equation  division,  157 
ETHICAL  OBLIGATION,  340 
Ethics,  343 

Eugenicist,  methods  of,  299 
Eugenical  predictions  uncertain,  305 

rules  as  to  defects,  recessive,  306 
serious,  306 
slight,  287,  306 
EUGENICS,  295 

contributory,  307 

declining  birthrate,  309 

definition  of,  296 

ideals,  297,  298 

negative,  302 

only  hope,  322 

positive,  305 

EUTHENICS,  249 

EVANS,  chromosomes  of  man,  163 
Evolution,  control  of,  291 

experimental,  215,  283 

progressive,  289 

promotion  of,  292,  343 


requires  new  characters,  274,  280, 
283 

retrogressive,  189 
EVOLUTION  OF  MAN,  287,  288 

contemporary,  287 

control  of,  291 

future,  289,  292 

intelligence  in,  291 

natural  selection  in,  292 

prehistoric,  287 
Exogastrula,  229 
Experience,  factor  in  behavior,  330 

learning  by,  49,  329 
Experimental  evolution,  283 

medicine,  4 

EXPERIMENTAL,    STUDY    OF    INHERI- 
TANCE, 82 
"Extracted"     Dominants     or     Reces- 

sives,  86 
Eyes,  development,  23 

color,  66,  119 

lacking,  231 

fused  together,  230 

Facial  features,  inheritance  of,  66 

Hapsburg  type,   119 
FACTORS  OF  DEVELOPMENT,  56-60 
Factors,  added  in  progressive  muta- 
tions, 283 

chemical    comparisons,    101,    130, 

283. 
definition  of,  100,  130 

differential,  101 

distribution    in    maturation     and 

fertilization,  180 
dominant      and      recessive,      not 

modified  by  union,  243 
drop  out  i-i  regressive  mutations, 

282 

extrinsic  and  intrinsic,  60 
for  color  developers,  102 
of  rabbits  and  mice,  103 
of  sweet  peas,  102,  104 
for  pigment,  102 
lethal,  105,  190 
location  in  cell,  130,  178,  iSr 
Mendelian,  101,  180 
modifying,  103 
multiple,  102 
nature  of,  101,  105,  130 
no  formation  de  novo,  283 
not  undeveloped  characters,  101 
origin  of  new,  282,  283 


Index 


369 


relations  to  characters,  180 

sex  determining,  165 
FAHLENBECK,  noble  families  of  Swe- 
den, 311 

Family  names,  extinction  of,  311 
FARADAY,  335 
Fat  stains,  effects  on  next  generation, 

112,  249 

"Fate  of  part,   function  of  position," 
236 

of  o'rganization,  237 
Fecundity,  inherited,  68 
Feeble-mindedness,  71,  117 
Feminist  movement,  340 
Fertility,  of  lower  types,  296 
FERTILIZATION,  n,  12,  133,  137 

a  stimulus  to  development,  133 

heterogeneous,  221 

union  of  germplasms,  134 
"Fewer  and  better  children,"  311 
Fluctuations,  74,  279 
FOL,  fertilization  of  starfish,  12 
Food,    influence    on    development    in 
tadpoles,  canaries,  bees,  230,  231 
FOOT  and  STROBELL,  on  chromosomes, 
182 

on  sex-limited  characters,  187 
FOREL,  effect  of  alcohol  on  germ  cells, 

221 
FORMATION  OF  SUBSTANCES  IN  CELLS, 

204 

FORMULAE,  INHERITANCE,  95 
Fowls,  races  of,  262,  263 

transplanted  ovaries,  241 
FREEDOM,  and  determinism,  327 

birth  and  growth  of,  330 

definition  of,  331 

development  of,  330 

from  reproduction,  340 

greatest  in  man,  330 

not  absolute,  327 

not  uncaused  activity,  331 

of  action,  53 

of  individual,  319,  339 

of   society,   339 
Friedrich's  Disease,  118 
Frog,  behavior  of,  330 

double  embryos,  222,  223 

early  development,  24,  25 
Function  and  structure  inseparable,  31 

correlated,  32,  68 

development  of,  29 


FUNCTIONAL  ACTIVITY,  231 

in  human  development,  250 
GALTON,  age  of  marriage,  307,  312 

"Ancestral  Inheritance,"  76,  78 

artistic  faculty,  64 

characters,  64 

definition  of  eugenics,  296 

diseases,  64,  76 

eugenical  policy,  300 

eye-color,  64,  76 

"Filial  Regression,"  79,  80 

genius  inherited,  71,  76 

heredity  vs.  civilization,  255 

intermarriage  of  scholars,  299 

kinds  of  inheritance,  73 

method  of,  63 

nature  and  nurture,  213 

on  Ancient  Greeks,  292,  293 

on  identical  twins,  253 

pioneer  in  heredity,  64 

poor  health  of  divines,  252 

religious    significance    of    evolu- 
tion, 343 

on  "sports,"  74 

statistical    study    of    inheritance, 
76-82 

stature,  64,  78,  80 

weight  of  seeds,  64 
Gamete,   10 

Gardener,  methods  of,  209 
Gastrula,  22,  25 

GATES,  Oenothera  chromosomes,  282 
GAUSS,  334 

"Gemmules,"  of  Darwin,  124,  129 
Generalized  types,  299 
Generations,  parental  and  filial,  84,  86 

symbols  of,  86 
Genes,  130 
Genius,  and  physical  defects,  252 

hereditary,  71 

unstable  nervous  organization,  71 
Genotype,  92,  95,  96,  97,  127,  272 
Geographical  isolation,  300 
Geotropism,  of  seedling,  42 
GERMAN  EMPEROR,  number  of  ances- 
tors, 78 
GERM  CELLS,  6,  133 

alive,  9 

and  body  cells,  125,  127 

complexity  of,  211 

possibilities  determined  in,  321 

potential  personalities  in,  312,  334 


370 


Index 


reactions  of,  38,  42,  329 

specific,  172,  177 
Germ  nuclei,  21 

Individuality  of  140,  141 
Germplasm,  in  nucleus,  127 

Theory  of  Weismann,  127,  238 
Germplasm  vs.  Somatoplasm,  126,  127 
"Germ  track,"  diagram  of  126,  149 
GERMINAL  BASES  OF  MIND,  36 
GERMINAL  CONTINUITY,  125 
Glandular  secretions,  effects  of,  232 
Glaucoma,  69,  118 
GODDARD,  feeble-mindedness  inherited, 

71 
GOLDSCHMIDT,  sex  determination,  169 

sex  hormones,  169 
Gonia,  148 

Grafts,  not  modified  by  stock,  241,  242 
Great  men,  in  crises,  335 
GREECE,  decay  of,  340,  34i 
GREEKS,  ancient,  293,  294 
Growth  period,  148 
GUDERNATSCH,  effects  of  food  on  tad- 

poles,  230 

Guinea-pigs,  recombinations  of  char- 
acters, 270,  271 

transplanted  ovaries,  241,  242,  243 
GUTHRIE,  transplanted  ovaries,  242 
GUYER,  on  chromosomes  of  man,  162 
GUYER  and  SMITH,  inheritance  of  in- 
duced eye  defects,  247,  248 
Gynandromorphs,  169 
Gypsy  moth,  sex  determination,  169 
HABITS,  definition,  250 

good  and  bad,  250,  330 
HAECKEL,  plastidules,  129 
HAEMOPHTLA,  68,  119,  188 
Hair,  color,  115,  116,  117 

form,  117 

white  forelock,  116 
Half  castes,  of  Australia,  301 

of  New  Zealand,  301 
Haploid  number  of  chromosomes,  156 
Hardships,   educational  value  of,  251 

335 
HARRIS,  induction  effects  of  poor  soil, 

246 
HARRISON,  on  transplanted  limbs,  236 

graft  of  tadpoles,  241,  242 
HARVEY,  epigram,  6 

epigenesis,  58 

HATSCHECK,    cleavage    and    gastrula- 
t-ion  of  AMPHIOXUS,  22 


Hereditary  lines,  interwoven,  342 
HEREDITARY  RESEMBLANCES,  65 

DIFFERENCES,  72 
Heredity,  and  memory,  245 

and  variation,  65,  75 

confused  ideas  of,  123 

definition  of,  132 

mechanism  of,  131,  133,  i/i 

more    potent    than    environment, 

313 

possible  improvements,  343 
theories  of,  133 
share  of  egg  and  sperm  in,  198 
usually    unchanged    by    environ- 
ment, 249,  321 

HEREDITY  AND  DEVELOPMENT,  132 
HEREDITY,    ENVIRONMENT,    TRAINING, 

253 

HERING,  Organic  memory,  45 
Heritage,  definition  of,  132 
Hermaphrodites,  168 
HERTWIG,   O.,  discovery  of   fertiliza- 
tion, 132 
human  ovum,  9 
idioblasts,  129 

influence  on  germ  cells  of  X- 
rays,  radium,  chemicals,  drug 
habit,  218,  221 

HERTWIG,  R.,  modification  of  sex  ra- 
tio, 166 
Heterosis,  274 
Heterozygosis,  273 
Heterozygotes,  84,  93,  96 
HIPPOCRATES,  124 
HOMO  SAPIENS,  287,  250 

NEANDERTHALENSIS,  289 
Homozygotes,  84,  03,  96 
"Homunculus,"  57 
HOPPE,    Effect    of    alcohol    on    germ 

cells,  221 

Hormones,  effects  of,  232 
Human  embryo,  development,  27,  28, 

30 
HUMAN  EVOLUTION,  CONTROL  OF,  292 

slow,  312 

Human  faculties,  definition,  250 
Human  Heredity,  no  improvement  in, 

292 

no  pure  lines,  114,  305 
Human    oosperm,   early   development, 

27,  28,  30 
ovum,  9,  10 
spermatozoa,  n 


Index 


371 


Humidity,  influence  on  mutation,  218 
Huntington's  Chorea,  118 
HUXLEY,  Evolution  and  Ethics,  255 
Hybrid  races,  quality  of,  301 
Hybridization,  273 

human,  301 

Hybrids,  increased  vigor,  273 
Hypertrophied    heart,    not    inherited, 

240 
Hypophysis,    effects    on    development, 

234 

Hypotrichosis,   117 
Hysteria,  71,  118 

Ideals,  individual  and  social,  298 
Identity,  sense  of,  54 
"Idioblasts"  of  Hertwig,  130 
Idioplasm,  of  Nageli,  128 
Immigration,  293,  301 

laws,  302 

Immunity,  transmission  of,  114 
Impulses,  conflicting,  50,  328 
Inbreeding,  299 
INDIVIDUAL  AND  RACE,  337 

minor  unit,  340 
INDIVIDUAL  CHARACTERS,  65 

Morphological,  66 

Physiological,  67 

Psychological,  70 

Teratological,  69 
INDIVIDUALS   AND   THEIR  CHARACTERS, 

63 

INDIVIDUALS  UNIQUE,  75 
"Induction,"    effect    of    colored    soil, 
247 

poor  soil,  cold,  alcohol,  246,  247 

not  inherited,  247 
Inequality  of  all  men,  320 
Infancy,  prolonged  in  man,  250,  323 
Infertility,  causes  of,  311 
INHERITANCE   OF   ACQUIRED    CHARAC- 
TERS, 237 

statement  of  problem,  239 

lack  of  evidence,  240 

no    influence   of    stock   on   graft, 
241 

dominants  and  recessives  remain 
pure,  243 

climatic  effects  not  inherited,  245 
Inheritance,  "alternative,"  73 

"blending,"  73 

of  baldness,  68 


cell  characters,  67 

dimples,  66 

facial  features,  66 

fecundity,  68 

genius,  71 

instincts,  70 

intellectual  capacity,  71 

left-handedness,   68 

longevity,  68 

moral  tendency,  71 

obesity,  68 

"particulate,"    73 

pathological  characters,  69 

physiological    characters,    67 

psychological  characters.  70 

sex-limited    and    sex-linked,    73, 
119,  171,  185 

stature,  66,  79,  80 

temperament,  71 

teratological  characters,  69 

through  cytoplasm,  195 

tuberculosis,  70 

will,  71 
INHERITANCE  FACTORS,  100,  129 

are  differential  causes,  100 

formulae,  95 

units,  129 

location  of,  130,  178 
Inheritance  material,  127 

seat  of,  131,  178 
Inheritance  units,  129 

location  of,  132 
Inhibition,  50,  330 
Insanity,  71,  117 
INSTINCTS,  39 

altruistic,  332 

inherited,  70 

origin  of,  42,  43, 

reproductive,  340 
INTELLECT,  45-49 
Intellectual  capacity,  inherited,  71 

genius,  71,  117 

mediocrity,  117 
Intelligence,    factor    in    control,    329, 

.331 

in  evolution  of  man,  292 
Intelligence,  from  Trial  and  Error,  48 
Interaction  of  parts,  231 
Internal  secretions,  effects  of,  232,  235 
Intersexes,  168 
Intra-cellular  pangenesis,  205 
INVERSE  SYMMETRY,  197,  799,  200,  201 


372 


Index 


Irritability,  9 

ISOLATION   OF   SUBSTANCES   IN   CELLS, 

205 
in  protozoa,  207 

JAMES,  WILLIAM,  304,  331 
JENNINGS,    action    of     selection,    on 
Paramecium,  269 

on  Difflugia,  269 

behavior  of  Paramecium,  47 

behavior  of  Stentor,  50 

inheritance    of    size    in    Parame- 
cium, 67 

on  Gallon's  law,  81 

on   potential   personalities,   334 

rapid    breeding    of    Paramecium, 
116 

training  of   star-fish,  51 
JEROME,   ST.,  33 

Jews,  mixture  with  Gentiles,  301 
JOHANNSEN,  action  of  selection,  269 

genotype  and  phenotype,   127 

inherited  weights  of  seeds,  66 

"pure  lines,"  266-269 
JOHNSON,    marriages   of    college   wo- 
men, 308 

KAMMERER,  effects  of  colored  soil  on 
salamanders,  247 

KEIBEL,  development  of  human  em- 
bryo, 27,  28,  30 

KEITH,  internal  secretions  and  racial 
characters,  235 

Keratosis,  117 

KING,  modification  of  sex  ratio,  166 

KORSCHELT  and  HEIDER,  Symmetry  of 
egg  of  Musca,  196 

LANE,  development  of  vision,  31 
LAMARCK,  on  inheritance  of  acquired 

characters,  237 

LAMARCKIAN  HYPOTHESIS,  245 
Lamarckism,  32 
LANG,  snail  hybrids,  106 
LAUGHLIN,    ancestors    and    contribu- 
tors, 79 

Laws  on  Eugenics,  303 
Learning  by  experience,  49,  330 
Left-handedness,  inheritance  of,  68 
Lens,  cataract,  66,  118 

development  of,  232 

displaced,  118 


weight  of,  66 

LEPTINOTARSA,  selection  in,  267 
Lethal  factors,  105 

sex-linked,  185 
Life,  artificial  production  of,  214,  215 

conditions  limited,  324 

definition  of,  6 

maze  of,  329 
LILLIE,  on  fertilizing,  173 

on  "free  martin,"  168 

sex  determination,  168 
Limbs,  transplanted,  236 
LINCOLN,  306 

LINKAGE  OF  CHARACTERS,  185 
LOCALIZATION  PATTERN,  197 

in  eggs  of  ctenophore,  flat-worm, 
echinoderm,        annelid-mollusk, 
chordate.  202 
Localization    of    substances    in    cells, 

142 

LOCKE,  JOHN,  214 

LOEB,    J.      Artificial    parthenogenesis, 
134,  174 

reflexes,  39 

tropisms,  43 

Logic,  as  test  of  truth,  323 
LOLIGO,  symmetry  of  the  egg,  196 
Longevity  inherited,  68 
LOTSY,  source  of  variations,  2/3,  274 
LOWELL,  BIGLOW  PAPERS.  237 
LUTZ,  chromosomes  of  Oenothera,  282 
Luxury,  cause  of  infertility,  313 

MACFARLANE,  on  Dionaea,  44 
McCLENDON,  production  of  one-eyed 

monsters,  172 
McCLUNG,  on  sex  determination,  161, 

180 

on  mammalian  chromosomes,  162 
McCRACKEN,,     maternal     inheritance, 

H3 
McDouGAL,  influence  of  chemicals  on 

ovules,  218 

MACDOWELL,  size  in  rabbits,  in 
Selection  in  Drosophila.  260 
MCGREGOR,   Restoration   of   Pithecan- 
thropus skull,  288 
Male  babies,  greater  mortality,  167 
MALTHUS,  theory  of,  303 
Man,  controls  destiny,  280 

dominant  races  of  289,  290 
evolution  of,  287 


Index 


373 


extermination  of,  290,  291 

extinct  types  of,  289 

freer  than  animals,  330 

mongrel  race,  306 

place  in  nature,  3 

prehistoric,    289 

races  of,  290 

slow  breeding  of,  114 

species  of,  287,  290 

value  of  races  of,  289 
MAORIS  of  New  Zealand,  290,  301 
Marriage,  age  of,  302,  310 

selection,  307,  308 
Marsupials,  26 

MASSART,  reactions  of  SPIRILLA,  39 
Materialism,  35 
"Maternal  impressions,"  29 
MATERNAL  INHERITANCE,  112,  203 
Matter  and  mind,  35 
MATURATION  PERIOD,  153 

divisions,  153,  154,  155 
"Mayflower"  descendants,  313 
Maze,  of  heredity,  76,  119 

life,  320 

MECHANISM  OF  DEVELOPMENT,  203 
MECHANISM  OF  HEREDITY,  132,  177 
Mechanistic  hypothesis,  330 
Mediocrity,  tendency  to,  78 
MEMORY,  43 

associative,  45 

organic,  45 
MENDEL,  abbot  of  Brunn,  82 

dominant    and    recessive    charac- 
ters, 84 

dominant  recessive  ratios,  84 

experiments   on   peas,   64,   83,   85 

inheritance  formulate,  95 

inheritance  units,  101 

method  of  work,  63,  83 

neglect  of  discoveries,  83 

purity  of  germ  cells,  88 

study  of  characters,  64 
MENDELIAN  ASSOCIATION  AND  DISSO- 
CIATION, 272 
Mendelian  factors  and  chromosomes, 

131,  179,  180,  181 

MENDELIAN    INHERITANCE,    diagrams 
of,  86,  87,  90 

IN  MAN,  114-119 

Table  of,  117-119 
MENDELIAN  PRINCIPLES,  98 

DOMINANCE,  99 


MODIFICATIONS   AND   EXTENSIONS, 

99 

SEGREGATION,  99 

UNIT  CHARACTERS,  98 
Mendelian  ratios,  simple,  84,  85 

other  ratios,  90,  91 

unusual  ratios,  108 

Monohybrid,  84,   91,  92 

dihybrid,  92,  93 

trihybrid,  95,  96,  97 

dominant-recessive,  84 

departures  from,  108 
MENDELISM,  82-119 
MENDELSSOHN,  reactions  of  Parame- 

cium,  40 

Meniere's  disease,  118 
Mentality,  influence  of  education,  214 
Mesoderm,  23 
Metabolism,  8 
METCHNIKOFF,   disharmonies  in  man, 

255 

Metempsychosis,  33 
Microscopic    particles,    smallest    visi- 
ble, 129 
MIDDLETON,    Effects   of    Selection    on 

Stylonychia,  269 

MIESHER,    On    stereoisomeres    of    al- 
bumin, 173 
Mind  and  body,  35 
Mind,  development  of,  32-56 
MIND,  GERMINAL  BASIS  OF,  36 
MIRABILIS,  white-red  cross,  87,  98,  106 
Mitosis,  16,  18,  19,  20,  136,  137 

significance  of,  131,  144,  148 
"Mneme"  theory,  245 
Modifiability  of  behavior,  50 
Molecular      constitution,       stereoiso- 
meres, 173 

Molecules,  largest  known,  129 
Monasticism,  295,  308 
Monohybrid,   84,  91,   92 
Monotremes,  26 
Monstrous  development,  217,  324 

cause  of,  217 
MONTGOMERY,    on    Chromosomes    of 

man,  162 

Moral  qualities   inherited,   71 
MORGAN,  gynandromorphs,  169 

chromosomes  of  Drosophila,  191 

"cross-overs,"  192,  193 

map  of  chromosomes,  194 

mutations  of  Drosophila,  185,  289, 


374 


Index 


284,  285,  286. 
sex  chromosome,  169,  170,  184 

186,  /p/ 

sex  determination  in  Phylloxera, 

169 
sex-linked  inheritance,     185,    186, 

187,  188,  189 

rapid  breeding  of  Drosophila,  116 
MORGAN  AND  BRIDGES,  modifying  fac- 
tors, 103 

lethal  factors,  105 

Drosophila  mutants,  284-286 
Morphological    characters,    66 

tests,  32 

Mosaic  development,  223 
Moth  and  flame,  330 
Mother   and   Child,  26 
MOTT,    insanity   inherited,    71 
Mouse,    maturation   and    fertilization, 

139 

Movements,    within    eggs    and   cleav- 
age cells,  38,  755,  136,  142,  232, 

effects  of  stopping,  232 

random,  43 

of  spermatozoa,  38 
Mulattoes,  skin  color,   109-111,   114 

in  Jamaica  301 

in  U.  S.,  301 

MULLER,  JOHANNES,  337 
MULLER,  mutations  of  Oenothera,  279 
MUSLOW,  154,  157 

Multiple   factors,  in  oats  and  wheat, 
109 

skin  color,  109-111 

size,  in,  112 
Multiple  sclerosis,  118 
MUSCA,  symmetry  of  egg,  796 
Muscular  atrophy,  118 
Mutation  Theory,  74,  274 
MUTATIONS,  73,  274 

AND  FLUCTUATIONS,  74,  274,  277, 

in  Drosophila,  280-286 

in  Leptinotarsa,  281 

in  Oenothera,  274,  286 

progressive  and  regressive,  282 

origin  of,  280 

Mutilations,  not  inherited,  240 
Myopia,  69 

NAGELI.  idioplasm,  130 

non-inheritance    of    alpine    habit, 

245 


Natural  selection,  266,  272,  291,  292, 

nullified,  294 
Nature,  definition  of,  321 

man  part  of,  321 

mechanistic  conception  of,  320 

vs.  nurture,  213 

stability  of,  289 

voluntaristic  conception  of,  319 
NECTURUS,  behavior  of,  51 
Neo-Darwinism,  35 
Neo-Lamarckism,  35,  245 
NETTLES  HIP,    hereditary    cataract,   66, 

67 

Neural  plate,  groove,  tube,  23 
Neuritis  optica,  sex-linked,  119 
Neuropathy,  118 

New  England  families  dying  out,  312 
XEWCOMB,  practical  eugenics,  298 
NEWTON,  335 

Night  blindness,  sex-linked,  118 
NILSON-EHLE,  multiple  factors,  106 
NON-MENDELIAN  INHERITANCE,  106 

107 

Notochord,  23 

Nuclear  division,  indirect,  see  Mitosis 
Nuclear  inheritance  theory,  179 
Nucleus,  does  not  differentiate,  145 

and  cytoplasm  concerned  in  her- 
edity, 203 

Nulliplex  character,  98 
Nutrition  and  Development,  232 

Obesity  inherited,  68 
OBSERVATIONS  ON  INHERITANCE,  63 
"Odd"  chromosome,  158  • 

OENOTHERA,  mutants,  274-279 

chromosomes  of,  282 
ONENESS  OF  LIFE,  3,  36 
Ontogeny  and  Phylogeny,  5 
Oocytes,  148 

of  rabbit,  750 

Oogenesis  of  Ancyracanthus,  755 
Oogonia,  148 
Oosperm,  13 

double   cell,    13 

individuality   of,    13 

infection  of,  114 
Organ-forming   substances,    59 

in  Styela.  Amphioxus,  frog,  142 
Organisms  of  humanity,  340 
Organization,  6,  8 
ORGANOGENY,  23 


Index 


375 


ORIGIN  OF  SEX  CELLS,  148 

DIVISION  PERIOD,  148 

Primitive  sex  cells,  148 
Oogonia,  148 

Spermatogonia,  148 

GROWTH  PERIOD,  148 

Oocytes,  148 

Spermatocytes,  148 

MATURATION  PERIOD,  153 
ORTHOGENESIS,  215 
OSBORN,  Cartwright  Lectures,  287 
Otosclerosis,  69,  118 
Ovaries,  transplanted,  241,  242 

OviPARITY,   26 

Oviparous  development,  13 
Ovules,   10 

Oxychromatin,     differential     distribu- 
tion, 206 
in  cell  body,  204 

PAINTER,  number  of  chromosomes  in 

man,  163,  164 

"Pangenes"  of  deVries,  129,  205 
Pangenesis,  hypothesis  of,  164,  237 
Panmerism,  128 
PARAMECITJM,   avoiding  reaction,  48 

behavior  of,  46 

reactions  to  heat  and  cold,  40 

races  differing  in  size,  67 

races  of,  66 

rapid  breeding  of,  116 

selection  in,  269 

trial  and  error,  47 
Parthenogenesis,  134 
"Particulate"    inheritance,    73 
Partition  walls  between  cells,  206 
PASTEUR,  335 
PATHOLOGICAL  CHARACTERS  INHERITED, 

69 

PEARL,  action  of  selection,  267 
PEARL  AND  PARSHLEY,  modification  of 

sex  ratio,  166 
PEARSON,  law  of  reversion,  77 

inheritance  of  tuberculosis,  69 

statistics,  fault  of,  81 
Peas,  Mendel's  experiments  on,  83 

monohybrids,  91,  92 

dihybrids,  92,  93 

trihybrids,  95,  96 

Permutations  in  distribution  of  chro- 
mosomes, 173,  175 


Personality,    determined   by   heredity, 
321,  324,  327 

development  of,  5,  56,  326 

infinity  of  chances  in,  307 

not  predetermined,  328 

potential,  334 

prediction  impossible,  307 
PHENOMENA  OF  DEVELOPMENT,  5 
Phenotype,  93,  95,  96,  97,  273 

vs.  genotype,  127 

Phototropism  of  Seedling,  41,  42 
PHYLLOXERA,   degeneration,   of   male- 
producing   spermatozoa,    167 
Physiological    characters,    inheritance 
of,  69 

division  of  labor,  31 

processes,  8-10 

states,  50,  330 

tests,  32,  173 

units,  129 
Pigeons,  behavior  of,  51,  52 

numerous  races,  260,  261 
PITHECANTHROPUS  ERECTUS,  skull,  289 
Pituitary  gland  influence  on  develop- 
ment, 234 

"Plasomes"  of  Wiesner,  129 
Plasticity  of  behavior,  51,  52 
"Plastidules"   of   Elsberg   and   Haec- 

kel,  129 

Plastosomes,  equal  division  of,  206 
PLATE,  ancestors  of  German  Emper- 
or, 78 

factors  for  coat  colors  of  mice, 
102 

Mendelian     inheritance     in     man, 

114-121 

PLATO,  on  transmigration,  33 
Polar  bodies,  n,  157 
Polarity,  197,  198 

of  Styela  egg,  140,  141,  143 
Polydactylism,  69,   116 
Polyhybrid,  92 
Population,  normally  stationary,  309 

of  Europe,  310 
Poultry,  selection  for  egg  production, 

267,  268 

PREFORMATION,  57,  177,  283,  323 
"Preinduction,"  in  Daphnia,  246 
"Preinheritance,"  113,  203 
Prepotency,  82 

PRESENCE  AND  ABSENCE  HYPOTHESIS, 
97 


376 


Primitive  sex  cells,  126,  148 

Principles  of  good  breeding,  violation 
of,  295 

Propagation  of  worst,  295 

PROTENOR,  sex  differentiation  in,  759 

Protoplasmic  and  Cellular  Organiza- 
tion, 6 

Psychical  Anlagen,  36 

Psychical    development,   table    of,    56 

PSYCHOLOGICAL  CHARACTERS  INHER- 
ITED, 70 

PUNNETT,  83,  103,  104 

"Pure    lines,"    266-269 

Puritans  and  Cavaliers  disappearing, 
3ii 

Purity  of  germ  cells,  88,  99 

PYTHAGORAS,  on  transmigration,  33 

Race  amalgamation,  320 

extermination,  290 

improvement,  4,  292,  340,  343 

preservation,  340 
Radium,   disintegration   of  atom,   283 

influence  on  spermatozoa,  218 
RANA,  grafted  tadpoles  of,  241,  242 
Reactions,  of  germ  cells,  38,  42 

machine-like,  330 
REASON,  45-49 
Reception  cone,  12,  136 
Recessive  characters,  84 

"extracted,"  86 

ratio  to  dominant,  84,  88 
Reduction  of  chromosomes,  155-157 
REFLEXES,  39,  40,  42 
Regeneration,  in  eggs  and  adults,  227 
Reproduction,  8,  30 
Responses,  useful,  46 

varied,  50 
Responsibility,  320,   331 

definition  of,  331 

of   society,  332,  339 

varied,  332 

Retinal  degeneration,  119 
RETZIUS,  human   spermatozoa,  9,  u 
Reversible  changes  not  inherited,  247, 

266 

Reversion,  73,  266 
RICHARDS,  mitosis  in  Fundulus,  20 
Rickets,  not  inherited,  239,  240 
RIDDLE,  Sex  determination,  166 
Rigidity  of  behavior,  51 
RIGNANO,   "Centro-epigenesis"   theory, 
245 


ROMANES,  260-265 

cattle,  265 

fowls,  262,  263 

pigeons,  260,  261 

swine,  264 

ROME,  decay  of,  338,  339 
ROSANOFF,  insanity  inherited,  71 
Rotifiers,  induction  effect  of  alcohol, 

246 
ROUSSEAU,  214 

Salamanders,   effects   of    colored   soil 

on,  247 

Savagery,  256,  287 
Scholars,  prize,  299 
SCHULTZE,  double  frog  embryos,  223 
Science,  duty  of,  337 
Segregation,  99 

apparent  lack  of,  108-112 

unusual  ratios,  108 

blending  of  color,  109 

blending  of  size,  in 

fundamental  to  Mendelism,  107 

of  Mendelian  factors,  99,  107 

of  substance  in  cells,  142,  206 
SELECTIVE  BREEDING,  only  method  of 
improving  race,  292 

Spartan  method,  292 
Selection,  value  in  Evolution,  266,  272 
Selective  Mating,  306 
Self-knowledge  and  Self-control,  335, 

343 

"Self  differentiation,"  235 
Self  discovery,  335 
SMITH,  ADAM,  214 
SEMON,  "Mneme"  theory,  245 
Sensitive  periods,  218 
SENSITIVITY,  37 

differential,  37 

general,  38 

of  embryo,  38 

of  germ  cells,  37 

Sex,  a  Mendelian  character,  91,   164, 
165 

influence   of    food    and   tempera- 
ture, 167 
Sex  cells,   fundamentally  alike,   10 

SEX   DETERMINATION,   162 

in  human  embryo,  166 
in  man,  162,  765 
in  Ascaris,  161 


Index 


377 


in  Protenor,  750 
in  Tenebrio,  160 
chromosomal  determination,  158 
XO  type,  158 
XY  type,  1 60 

Environmental  influence,   166 
alteration  of  sex  ratios,  166 

hermaphrodites      and      inter- 
sexes,   168 
McCLUNG  on,  159 
WILSON  on,  160,  161 
STEVENS  on,  160 
Sex   glands,   effects   on   development, 

232 

Sex  hormones,  173 
SEX-LIMITED  INHERITANCE,  73,  170 
SEX-LINKED     INHERITANCE,     73,     119, 

185-189 

Sex  ratio,  modification  of,  166-168 
Sexual  reproduction,  value  of,  174 
Share  of  egg  and  sperm  in  heredity, 

198-203 

Share    of    chromosomes    and    cyto- 
plasm, 203 
SHULL,  A.   F.,   sex  determination  in 

Rotifers,  168 

SHULL,  G.  H.,  leaf  colors,  113 
heterosis,  274 
Oenothera  mutants,  277 
Significance  of  cleavage,  144,  148,  206 

of  mitosis,  131,  145,  150 
Simplex  character,  98 
Skin  color,  109 

mulatto,  109,  no 
influence  of  light  on,  214 
Slow  breeding  of  man,  116,  315 
SOBOTTA,   fertilization   of   mouse,    139 
Social  inheritance  vs.  germinal,  254 
Social  institutions,  deal  only  with  en- 
vironment, 254 

Society,   highest    grade    of    organiza- 
tion, 339 
power  of,  295 
responsibility  of,  339 
supreme  duty  of,  340-342 
Soil,  poor,  induction  effect  on  plants, 
246 

SOMAT'C  DISCONTINUITY,  125 

Somatoplasm,  in  cell  body,  127 
SPARTA,  destruction  of  unfit,  292 
Special  senses,  origin  of,  38 


Specialized  types,  209 

SPECIFICITY  OF  GERM  CELLS,  171,  177 

of  protoplasm,  172 
SPENCER,  physiological  units,  129 
Sperm  centrosome,  136 
Sperm  nucleus,  II,  21 
Spermatocytes,  148 
Spermatogenesis    of    Ancyracanthus, 

154 

Spermatogonia,   148 
Spermatozoon,  p,  //,  12 

formation  of,  157 
Spina  bifida,  228 
Spinal  cord,  development,  23 
Spindle,  mitotic,  17,  18,  138 
SPIRILLA,  reactions  to  chemicals,  30 
"Sports,"  74 

Star-fish,  isolated  cleavage  cells,  224 
Star-fish,  isolated  cleavage  cells,  222 
Statistical  methods  vs.  Physiological, 

80,  8 1 
STATISTICAL   STUDY  OF  INHERITANCE. 

76-81 
Stature,  inheritance  of,  66,  78,  80 

tendency  to  mediocrity,  78 

influence  of  food  on,  216 
STENTOR,  modifiable   behavior,   50 
Sterile  insects,  315 
Sterility,  292,  311,  313 
Sterilization,  303 

STEVENS,  on  sex  determination,  160 
Stimuli,  chemical  and  physical,  217 

331 

conflicting,  50 
definition,  50,  216 
DEVELOPMENTAL,  216 
non-specific,  217 
external  and  internal,  50,  330 
range  of,  331 

rational,  social,  ethical,  330 
summation  of,  43 
STOCKARD,     alcohol     on     guinea-pigs, 

218,  219,  220 
experimental    cyclopia,    172,    229, 

230 

STRASBURGER,   phototropism   of    seed- 
ling, 41 
Structures    and    functions,    reciprocal 

relations,  31,  35,  68 
STYELA.  anterior  half -embryos,  226 
dislocated  organs,  228 
egg  substances,  140 


378 


Index 


gastrulation  and  larva,  147 

half   and   three-quarter   embryos, 
224 

maturation,       fertilization       and 
cleavage,  143,  148,  149 

posterior  half-embryos,  226 
SUMNER,  effect  of  cold  on  mice,  246 
Superman,  298,  299,  316 
SWEDEN,  extinct  noble  families,  312 
Swine,  wild  and  domestic,  264 
SYMMETRY,  196,  109 
Synapsis,  150,  154 
Syndactylism,  69,  117 

Tadpoles,    fed    on    thyroid,    thymus, 

adrenal,  230 
Talents,  unused,  333 

parable  of,  334 

Temperament,  inheritance  of,  71,  321 
Temperature,   influence  on   mutation, 

218 

influence  on  cell  division,  217 
TENEBRIO,  sex  differentiation  in,  160 
TENNENT,     modified     dominance     in 

echinoderm  hybrids,  105 
Teratological  characters  inherited,  69 
TERTULLIAN,  33 
Tetrads,  153 
THOMPSON,  cross  of  yellow  X  green 

peas,  85 

diagram  of  Galton's  1st  Law,  78 
Thomsen's  disease,  118 
THORNDTKE,   behavior    of    dogs,    cats, 

monkeys,  48 
"Thoroughbreds,"  300 
Thymus  gland,  fed  to  tadpoles,  230 
Thyroid,  effects  on  development,  233 

gland,  fed  to  tadpoles,  230 
Tissue  cells,  7 
Totipotence,    of    cleavage    cells,    222, 

226,  236 
Tower,  action  of  selection,  267 

mutations    in    Leptinotarsa,    218, 

281 
TOXOPNEUSTES,    fertilization    of    egg, 

12 

Toyama,    maternal    inheritance,    113 
Traducianism,  33 
Training  of  animals,  51 
Transmission  hypothesis,  124 
Trial  and  Error,  46 


Trihybrid,  95,   96 
Triplets,  hereditary,  68 
Trophic  correlations,  232 
TROPISMS,  39,  42 

TSCHERMAK,    rediscovery    of    "Men- 
del's Law,"  82 

Tuberculosis,  inheritance  of,  70 
Turbellarian  type  of  egg,  202 
Twins,  fraternal,  228 

hereditary,  68 

identical,  75,  ^29,  252,  325 
Ultra-microscopic  units,  129 
Uniqueness    of    every    individual,    75, 

177 

UNIT  CHARACTERS,  98,  100 
UNITS  OF  LIVING  MATTER,  129 

ultra  microscopic,  129 

units  of  growth  and  division,  129 

units  of  heredity,  100-106,  129-131 
Unity  of  organism,  55 
Universal  laws,  323 
Use  and  disuse,  effects  of,  232 

effects  not  inherited,  237 
Useful  responses,  46 
Uterus,  attachment  of  oosperm  to,  27 

Variability,    caused    by    environment, 

280 
Variations,  72 

continuous,  74 

discontinuous,  74 

fluctuations  74,  280 

meristic,  74 

mutations.  74,  280-286 

"sports,"   74 
Varied  responses,  51 
Vegetative  pole  of  egg,  n,  195 
VIVIPARITY,  26 
Viviparous  development,  26 

WALTER,    diagram    of    Galtonian    in- 
heritance, 73 
filial    regression,   80 

Wars,  effects  of,  303 

shake  off  social  heredity,  256 

Wasserman  test,  173 

WATASE,    symmetry    of    egg    of    Lo- 
ligo,  796 

WEEKS,  71 

Weidal  test,  173 

WEISMANN,    determinants    and    bio- 
phores,  129 


Index 


379 


germplasm  theory,  127,  130 
hereditary      and      environmental 

variations,  75 

on   differential   division   of    chro- 
mosomes,   145,  204 
inheritance    of    acquired    charac- 
ters, 238 

nuclear     control     of     differentia- 
tion, 204 

reduction  of  chromosomes  157 
WENRICH,   chromosomes   of   Phryno- 

tettix,  152 
What  is  Life?,  6 

WHITMAN,  behavior  of  CLEPSINE,  51 
NECTURUS,  51 
pigeons,   51,   52 
freedom  and  choice,  52,  53 
sex  of  pigeons,  169 

WHITNEY,   induction  effects  of  alco- 
hol, 246 
sex     determination     in     Rotifers, 

1 66 

WIEMAN,  chromosomes  of  man,  163 
WIESNER,  plasomes,  129 
WILDER,  Duplicate  twins  and   double 

monsters,  229 
WILL,  50-53 

absolutely  free,  320,  331 

denned,  331 

good  and  evil,  320 


inherited,  71,  334 

nature,  expression  of,  319 

responsibility  and,  331 

supreme  faculty,  333 

training  of,  333 
WILSON,  cell  division,  18,  19 

distribution  of  factors,  180 

of  chromosomes,  181 

dwarf     and     double     Amphioxus 
embryo,  222 

fertilization  of  sea-urchin,  12 

on  sex  determination,  159,  161 
WINIWARTER,     on     chromosomes     of 
man,  162 

oocytes  of  rabbit,  750 
WOLFF,  "Theoria  Generationis,"  58 
WOLTERECK,  induction  effects  of,  246 

preinduction,  246 

Women's  Colleges,  influence  on  mar- 
riage, 309 
WOODS,  "Heredity  in  Royalty,"  71 

X-rays,  influence  on  spermatozoa,  218 
X  and  Y  chromosomes,  170,  184 

Yolk  influence  on  size  of  egg,  10 

ZELENY   AND   MATTOON,    Selection   in 

Drosophila,  269 
Zygote,    10 


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